Harden security, polish admin UI and document Windows WebDAV

- helmet, express-rate-limit (login/setup/customer-auth/me-password)
- Constant-time login (bcrypt always runs against a dummy hash on miss)
- Cookie secure flag follows req.protocol; trust proxy is env-gated to
  prevent X-Forwarded-For spoofing on direct exposure
- Drop SVG from accepted logo types (same-origin XSS) and resolve the
  served logo path against LOGO_DIR as defense in depth
- Self-service /me/password endpoint plus header button; bumps minimum
  password length to 8 across backend, prompts and edit modal
- Multer 1.x → 2.x for current security backports
- Customer edit modal replaces stacked prompts; user role is now an
  inline dropdown with a confirm-and-revert flow
- Windows .reg helper plus README section for Basic-Auth-over-HTTP and
  the http:// vs \\HOST@PORT\DavWWWRoot\ mapping syntax

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

windows/enable-webdav-basicauth.reg

@@ -0,0 +1,12 @@
+Windows Registry Editor Version 5.00
+
+; Enables WebDAV via Basic Auth over plain HTTP and lifts the 50 MB
+; file size limit so large uploads/downloads work over a mapped drive.
+;
+; Rechtsklick -> "Zusammenführen" (als Administrator), dann
+; den WebClient-Dienst neu starten:
+;     net stop WebClient && net start WebClient
+
+[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\WebClient\Parameters]
+"BasicAuthLevel"=dword:00000002
+"FileSizeLimitInBytes"=dword:ffffffff