fix: "Anzeigen"-Buttons öffnen Datei wieder im Browser-Tab

Folge-Symptom des Pen-30.13-Fixes: alle file-downloads liefen mit
Content-Disposition: attachment – das ist gegen Stored-XSS richtig,
hat aber die "Anzeigen"-Buttons (Bankkarten / Ausweise /
Verträge / etc.) kaputtgemacht, weil der Browser jetzt
herunterlud statt im Tab zu öffnen.

Magic-Byte-basierter Whitelist-Pfad eingebaut: optional ?disposition=
inline am Download-Endpoint, ABER nur wenn die ersten Bytes der
Datei das Magic eines safe Typs zeigen (PDF, PNG, JPEG, GIF, WebP).
Bei Mismatch fällt's auf attachment zurück – Stored-XSS bleibt
weiterhin unmöglich, falls jemand HTML als .pdf hochlädt.

Frontend: neuer viewUrl(path)-Alias = fileUrl(path, {inline: true}).
Alle Stellen mit `<a href={fileUrl(...)} target="_blank">` oder
`window.open(fileUrl(...), '_blank')` (13 Stellen über CustomerDetail,
ContractDetail, PdfTemplates, GDPRDashboard, InvoicesSection)
nutzen jetzt viewUrl. Download-Stellen bleiben fileUrl
(= attachment, byte-genaues File-Save).

Live-verifiziert auf dev:
- ohne Param: attachment (default, Stored-XSS-Schutz)
- ?disposition=inline + echte PDF: inline + application/pdf
- ?disposition=inline + HTML als .pdf: attachment (Magic-Mismatch
  → Browser lädt herunter statt zu rendern)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/pages/contracts/ContractDetail.tsx

@@ -20,7 +20,7 @@ import CopyButton, { CopyableBlock } from '../../components/ui/CopyButton';
 import { formatDate } from '../../utils/dateFormat';
 import { useProviderSettings } from '../../hooks/useProviderSettings';
 import type { ContractType, ContractStatus, SimCard, MeterReading, ContractTask, ContractTaskSubtask, ContractMeter, ContractDocument } from '../../types';
-import { fileUrl } from '../../utils/fileUrl';
+import { fileUrl, viewUrl } from '../../utils/fileUrl';
 
 const typeLabels: Record<ContractType, string> = {
   ELECTRICITY: 'Strom',
@@ -2118,7 +2118,7 @@ export default function ContractDetail() {
                   {c.cancellationLetterPath ? (
                     <div className="flex items-center gap-3 flex-wrap">
                       <a
-                        href={fileUrl(c.cancellationLetterPath)}
+                        href={viewUrl(c.cancellationLetterPath)}
                         target="_blank"
                         rel="noopener noreferrer"
                         className="text-blue-600 hover:underline text-sm flex items-center gap-1"
@@ -2175,7 +2175,7 @@ export default function ContractDetail() {
                     <>
                       <div className="flex items-center gap-3 flex-wrap">
                         <a
-                          href={fileUrl(c.cancellationConfirmationPath)}
+                          href={viewUrl(c.cancellationConfirmationPath)}
                           target="_blank"
                           rel="noopener noreferrer"
                           className="text-blue-600 hover:underline text-sm flex items-center gap-1"
@@ -2261,7 +2261,7 @@ export default function ContractDetail() {
                   {c.cancellationLetterOptionsPath ? (
                     <div className="flex items-center gap-3 flex-wrap">
                       <a
-                        href={fileUrl(c.cancellationLetterOptionsPath)}
+                        href={viewUrl(c.cancellationLetterOptionsPath)}
                         target="_blank"
                         rel="noopener noreferrer"
                         className="text-blue-600 hover:underline text-sm flex items-center gap-1"
@@ -2318,7 +2318,7 @@ export default function ContractDetail() {
                     <>
                       <div className="flex items-center gap-3 flex-wrap">
                         <a
-                          href={fileUrl(c.cancellationConfirmationOptionsPath)}
+                          href={viewUrl(c.cancellationConfirmationOptionsPath)}
                           target="_blank"
                           rel="noopener noreferrer"
                           className="text-blue-600 hover:underline text-sm flex items-center gap-1"
@@ -3484,7 +3484,7 @@ function ContractDocumentsSection({
                       {doc.documentType}
                     </span>
                     <a
-                      href={fileUrl(doc.documentPath)}
+                      href={viewUrl(doc.documentPath)}
                       target="_blank"
                       rel="noopener noreferrer"
                       className="text-sm text-blue-600 hover:underline"

frontend/src/pages/customers/CustomerDetail.tsx

@@ -21,7 +21,7 @@ import { formatDate } from '../../utils/dateFormat';
 import { getContractTypeInfo } from '../../utils/contractInfo';
 import { useProviderSettings } from '../../hooks/useProviderSettings';
 import type { Address, BankCard, IdentityDocument, Meter, Customer, CustomerRepresentative, CustomerSummary, CustomerConsent, ConsentType, ConsentStatus, RepresentativeAuthorization } from '../../types';
-import { fileUrl } from '../../utils/fileUrl';
+import { fileUrl, viewUrl } from '../../utils/fileUrl';
 
 export default function CustomerDetail({ portalCustomerId }: { portalCustomerId?: number } = {}) {
   const { id } = useParams();
@@ -577,7 +577,7 @@ function BusinessDataCard({
           {customer.businessRegistrationPath ? (
             <div className="flex items-center gap-2 flex-wrap">
               <a
-                href={fileUrl(customer.businessRegistrationPath)}
+                href={viewUrl(customer.businessRegistrationPath)}
                 target="_blank"
                 rel="noopener noreferrer"
                 className="text-blue-600 hover:underline text-sm flex items-center gap-1"
@@ -628,7 +628,7 @@ function BusinessDataCard({
           {customer.commercialRegisterPath ? (
             <div className="flex items-center gap-2 flex-wrap">
               <a
-                href={fileUrl(customer.commercialRegisterPath)}
+                href={viewUrl(customer.commercialRegisterPath)}
                 target="_blank"
                 rel="noopener noreferrer"
                 className="text-blue-600 hover:underline text-sm flex items-center gap-1"
@@ -948,7 +948,7 @@ function BankCardsTab({
                 {card.documentPath ? (
                   <div className="flex items-center gap-2 flex-wrap">
                     <a
-                      href={fileUrl(card.documentPath)}
+                      href={viewUrl(card.documentPath)}
                       target="_blank"
                       rel="noopener noreferrer"
                       className="text-blue-600 hover:underline text-sm flex items-center gap-1"
@@ -1184,7 +1184,7 @@ function DocumentsTab({
                 {doc.documentPath ? (
                   <div className="flex items-center gap-2 flex-wrap">
                     <a
-                      href={fileUrl(doc.documentPath)}
+                      href={viewUrl(doc.documentPath)}
                       target="_blank"
                       rel="noopener noreferrer"
                       className="text-blue-600 hover:underline text-sm flex items-center gap-1"
@@ -4123,7 +4123,7 @@ function ConsentTab({
           {customer.privacyPolicyPath ? (
             <div className="flex items-center gap-3 flex-wrap">
               <a
-                href={fileUrl(customer.privacyPolicyPath)}
+                href={viewUrl(customer.privacyPolicyPath)}
                 target="_blank"
                 rel="noopener noreferrer"
                 className="text-blue-600 hover:underline text-sm flex items-center gap-1"
@@ -4441,7 +4441,7 @@ function AuthorizationsSection({ customerId, customerEmail }: { customerId: numb
               {auth.documentPath ? (
                 <>
                   <a
-                    href={fileUrl(auth.documentPath)}
+                    href={viewUrl(auth.documentPath)}
                     target="_blank"
                     rel="noopener noreferrer"
                     className="text-blue-600 hover:underline text-xs flex items-center gap-1"

frontend/src/pages/settings/GDPRDashboard.tsx

@@ -7,7 +7,7 @@ import Card from '../../components/ui/Card';
 import Button from '../../components/ui/Button';
 import Select from '../../components/ui/Select';
 import { ArrowLeft, FileText, Users, CheckCircle, Clock, XCircle, AlertTriangle, Download, X, ChevronRight } from 'lucide-react';
-import { fileUrl } from '../../utils/fileUrl';
+import { viewUrl } from '../../utils/fileUrl';
 import { useAuth } from '../../context/AuthContext';
 
 const STATUS_OPTIONS = [
@@ -364,7 +364,7 @@ export default function GDPRDashboard() {
                         <Button
                           variant="ghost"
                           size="sm"
-                          onClick={() => window.open(fileUrl(`/uploads/${request.proofDocument}`), '_blank')}
+                          onClick={() => window.open(viewUrl(`/uploads/${request.proofDocument}`), '_blank')}
                           title="Löschnachweis anzeigen"
                         >
                           <FileText className="w-4 h-4 text-blue-500" />

frontend/src/pages/settings/PdfTemplates.tsx

@@ -9,7 +9,7 @@ import Input from '../../components/ui/Input';
 import Badge from '../../components/ui/Badge';
 import Modal from '../../components/ui/Modal';
 import { ArrowLeft, Plus, Edit, Trash2, FileText, Upload, Link2, Eye, Play } from 'lucide-react';
-import { fileUrl } from '../../utils/fileUrl';
+import { viewUrl } from '../../utils/fileUrl';
 
 export default function PdfTemplates() {
   const navigate = useNavigate();
@@ -96,7 +96,7 @@ export default function PdfTemplates() {
                   <Button variant="ghost" size="sm" onClick={() => setTestTemplate(t)} title="Testvorschau mit Vertragsdaten">
                     <Play className="w-4 h-4 text-green-500" />
                   </Button>
-                  <a href={fileUrl(t.templatePath)} target="_blank" rel="noopener noreferrer">
+                  <a href={viewUrl(t.templatePath)} target="_blank" rel="noopener noreferrer">
                     <Button variant="ghost" size="sm" title="Leere Vorlage anzeigen">
                       <Eye className="w-4 h-4" />
                     </Button>