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)

frontend/nginx.conf

@@ -29,6 +29,26 @@ server {
         proxy_set_header X-Forwarded-Proto $scheme;
     }
 
+    # MIME types: the pdf.js worker ships as .mjs, and a browser REFUSES to
+    # start a module worker served as application/octet-stream (nginx's
+    # default for unknown extensions) — which crashes the PDF preview page.
+    # Declared explicitly rather than relying on the system mime.types.
+    types {
+        text/javascript    mjs js;
+        text/css           css;
+        text/html          html;
+        image/svg+xml      svg;
+        image/png          png;
+        image/jpeg         jpg jpeg;
+        image/webp         webp;
+        image/x-icon       ico;
+        font/woff2         woff2;
+        font/woff          woff;
+        application/json   json;
+        application/pdf    pdf;
+        application/wasm   wasm;
+    }
+
     # SPA fallback — all client-side routes go to index.html
     location / {
         try_files $uri $uri/ /index.html;