feat(files): preview attachments via short-lived signed URLs

Downloads require an Authorization header, which <iframe>/<img> cannot
send. The previous blob: workaround broke under download managers (IDM
hooks blob URLs, fails to resolve them, and leaves a 0-byte file) and in
Chrome's mobile emulation, which has no inline PDF plugin.

Backend:
- preview_token.py: HMAC-signed token bound to one (quote, kind, file)
  with a 5-minute TTL, keyed off an existing secret
- GET /quotes/{id}/preview-url/{kind}/{attachment} mints the URL
- the two file GET routes accept a Bearer session OR the signed ?t= token
  (they leave the global auth dep, so the other six file routes now carry
  it explicitly — nothing is open by omission)

Frontend:
- api.files.previewUrl() replaces the blob helper
- shared FilePreviewModal + useFilePreview; FileList and the dashboard
  quote card both use it
- PDFs render with the browser's native viewer on desktop and pdf.js on
  canvas where no inline viewer exists (mobile); pdf.js is fed the bytes
  in memory so no PDF URL is exposed for a download manager to intercept
- removed the per-attachment download buttons on the dashboard card and
  the modal header

Ops:
- nginx: explicit MIME types (.mjs as text/javascript) — a module worker
  served as octet-stream is refused and crashed the page
- frontend service bypasses the nginx entrypoint, whose apk manifest call
  hangs when the package mirror is unreachable (502s)

backend/app/main.py

@@ -227,10 +227,16 @@ async def unhandled_error_handler(_request: Request, exc: Exception):
 # pm, users) keep them — the global dependency guarantees nothing is open
 # by omission.
 from app.routers.auth import router as _auth_router  # noqa: E402
+# The files router enforces its own access check per route: a browser cannot
+# attach an Authorization header to <iframe>/<img> loads, so the two file GET
+# routes accept a short-lived signed ?t= token as well as a Bearer session.
+from app.routers.files import router as _files_router  # noqa: E402
 
 for router in ROUTERS:
     if router is _auth_router:
         app.include_router(router, prefix=API_V1_PREFIX)
+    elif router is _files_router:
+        app.include_router(router, prefix=API_V1_PREFIX)
     else:
         app.include_router(router, prefix=API_V1_PREFIX, dependencies=[Depends(get_current_user)])