feat: Office-Preview im neuen Tab + DOCX/XLSX/Text bearbeitbar

Preview-System komplett ueberarbeitet:
- Neuer Tab: Doppelklick oder Auge-Icon oeffnet Vorschau im neuen Tab
- Dedizierte PreviewView mit Toolbar (Zurueck, Bearbeiten, Speichern, Download)
- Token wird als Query-Parameter an Preview/Download-URLs angehaengt (kein 404 mehr)

Unterstuetzte Formate:
- PDF: Inline-Anzeige im iFrame
- Bilder: Zentrierte Anzeige mit Schatten
- DOCX: HTML-Darstellung mit Formatierung (Headings, Bold, Italic, Tabellen)
- XLSX: Tabellen-Ansicht mit Sheet-Tabs
- PPTX: Folien-Navigation (vor/zurueck)
- Text/Code: Monospace mit Syntax

Bearbeitung (neu!):
- DOCX: ContentEditable-Editor, Bold/Italic/Headings bleiben erhalten,
  Speichern schreibt zurueck als .docx (python-docx)
- XLSX: Direkt in der Tabelle bearbeiten (Zellen anklicken),
  Speichern schreibt zurueck als .xlsx (openpyxl)
- Text/Code: Textarea-Editor, Speichern als UTF-8

Backend: POST /files/<id>/save mit type-spezifischer Konvertierung
- html -> DOCX (Headings, Bold/Italic/Underline erhalten)
- spreadsheet -> XLSX (Zahlen werden automatisch konvertiert)
- text -> direkt als Datei

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/office.py

@@ -1,4 +1,7 @@
 import io
+import os
+import hashlib
+from datetime import datetime, timezone
 from pathlib import Path
 
 from flask import request, jsonify, current_app, send_file
@@ -168,3 +171,146 @@ def _convert_pptx(filepath):
             'html': '\n'.join(content_parts) if content_parts else '<p>(Leere Folie)</p>',
         })
     return slides
+
+
+# ========== Save (write back edited documents) ==========
+
+@api_bp.route('/files/<int:file_id>/save', methods=['POST'])
+@token_required
+def save_file(file_id):
+    """Save edited content back to the original file format."""
+    user = request.current_user
+    f, err = _get_file_or_403(file_id, user, 'write')
+    if err:
+        return err
+
+    if f.is_folder:
+        return jsonify({'error': 'Ordner koennen nicht gespeichert werden'}), 400
+
+    data = request.get_json()
+    save_type = data.get('type', '')
+    filepath = Path(current_app.config['UPLOAD_PATH']) / str(f.owner_id) / f.storage_path
+
+    try:
+        if save_type == 'html' and f.name.endswith('.docx'):
+            _save_html_to_docx(filepath, data.get('content', ''))
+        elif save_type == 'spreadsheet' and (f.name.endswith('.xlsx') or f.name.endswith('.xls')):
+            _save_sheets_to_xlsx(filepath, data.get('sheets', []))
+        elif save_type == 'text':
+            filepath.write_text(data.get('content', ''), encoding='utf-8')
+        else:
+            return jsonify({'error': f'Speichern fuer diesen Typ nicht unterstuetzt'}), 400
+
+        # Update file metadata
+        f.size = os.path.getsize(str(filepath))
+        h = hashlib.sha256()
+        with open(str(filepath), 'rb') as fh:
+            for chunk in iter(lambda: fh.read(8192), b''):
+                h.update(chunk)
+        f.checksum = h.hexdigest()
+        f.updated_at = datetime.now(timezone.utc)
+        db.session.commit()
+
+        return jsonify({'message': 'Gespeichert', 'size': f.size}), 200
+    except Exception as e:
+        return jsonify({'error': f'Speichern fehlgeschlagen: {str(e)}'}), 500
+
+
+def _save_html_to_docx(filepath, html_content):
+    """Convert HTML content back to DOCX."""
+    from docx import Document
+    from docx.shared import Pt
+    import re
+
+    doc = Document()
+
+    # Simple HTML to DOCX conversion
+    # Strip tags and convert basic elements
+    html = html_content.replace('\r\n', '\n').replace('\r', '\n')
+
+    # Process block elements
+    blocks = re.split(r'<(?:p|h[1-3]|br\s*/?)(?:\s[^>]*)?>|</(?:p|h[1-3])>', html)
+
+    # Find tag types
+    tags = re.findall(r'<(/?(?:p|h[1-3]|br\s*/?)(?:\s[^>]*)?)>', html)
+
+    current_tag = 'p'
+    for i, block in enumerate(blocks):
+        text = re.sub(r'<[^>]+>', '', block).strip()
+        if not text:
+            if i < len(tags):
+                tag = tags[i] if i < len(tags) else ''
+                if tag.startswith('h1'):
+                    current_tag = 'h1'
+                elif tag.startswith('h2'):
+                    current_tag = 'h2'
+                elif tag.startswith('h3'):
+                    current_tag = 'h3'
+                else:
+                    current_tag = 'p'
+            continue
+
+        if current_tag == 'h1':
+            doc.add_heading(text, level=1)
+        elif current_tag == 'h2':
+            doc.add_heading(text, level=2)
+        elif current_tag == 'h3':
+            doc.add_heading(text, level=3)
+        else:
+            # Check for bold/italic in remaining inline tags
+            para = doc.add_paragraph()
+            # Simple inline parsing
+            parts = re.split(r'(</?(?:strong|b|em|i|u)>)', block)
+            bold = False
+            italic = False
+            underline = False
+            for part in parts:
+                if part in ('<strong>', '<b>'):
+                    bold = True
+                elif part in ('</strong>', '</b>'):
+                    bold = False
+                elif part in ('<em>', '<i>'):
+                    italic = True
+                elif part in ('</em>', '</i>'):
+                    italic = False
+                elif part in ('<u>',):
+                    underline = True
+                elif part in ('</u>',):
+                    underline = False
+                else:
+                    clean = re.sub(r'<[^>]+>', '', part)
+                    if clean:
+                        run = para.add_run(clean)
+                        run.bold = bold
+                        run.italic = italic
+                        run.underline = underline
+
+        current_tag = 'p'
+
+    doc.save(str(filepath))
+
+
+def _save_sheets_to_xlsx(filepath, sheets_data):
+    """Save spreadsheet data back to XLSX."""
+    from openpyxl import Workbook
+
+    wb = Workbook()
+    # Remove default sheet
+    wb.remove(wb.active)
+
+    for sheet_data in sheets_data:
+        ws = wb.create_sheet(title=sheet_data.get('name', 'Sheet'))
+        for ri, row in enumerate(sheet_data.get('rows', []), 1):
+            for ci, cell_value in enumerate(row, 1):
+                val = cell_value
+                # Try to convert to number
+                try:
+                    if '.' in str(val):
+                        val = float(val)
+                    else:
+                        val = int(val)
+                except (ValueError, TypeError):
+                    pass
+                ws.cell(row=ri, column=ci, value=val if val != '' else None)
+
+    wb.save(str(filepath))