feat(files): a file that is not media opens in a viewer — an image, a PDF, text as text

- GET /api/files/{id}/preview: an allow-list on the stored extension (downloads/preview.py) picks
  image / PDF / text; served inline under `CSP: sandbox; frame-ancestors 'self'`, the type from the
  table; HTML and SVG arrive as plain text; a text is its first 1 MiB (X-Preview-Truncated), sent
  as UTF-8 whatever it was written in (BOM, UTF-8, else cp1250); a NUL means binary: 415
- the list says which preview each file gets (asset.preview); FE/BE contract on kinds and header
- the viewer (shared dialog shell): ← / → through the list's such files in its order, Save, open in
  a tab, the file's menu; one stage height for every kind; a no-preview card offers the file
- the name, the card's picture and the menu's Preview open it; an image up to 8 MB is its own
  thumbnail

backend/app/main.py

@@ -268,7 +268,8 @@ async def static_cache_headers(request, call_next):
 # Baseline security response headers (SEC-1). A minimal, safe set that needs no per-route tuning:
 # nosniff stops MIME-confusion, the Referrer-Policy keeps the (private) app URL from leaking to
 # external sites the user follows, `frame-ancestors 'none'` blocks click-jacking by refusing to be
-# iframed anywhere (we never embed our own pages; we embed OTHERS, which this does not affect), and
+# iframed anywhere (we embed OTHERS, which this does not affect; the one own answer the app frames, a
+# library file's preview, sets its own `frame-ancestors 'self'` — these are defaults, `setdefault`), and
 # Permissions-Policy turns off powerful features the app never uses. `X-Frame-Options: DENY` duplicates
 # the frame-ancestors intent for legacy engines that never implemented CSP frame-ancestors (belt and
 # suspenders — modern browsers use the CSP directive and ignore the header). A full Content-Security-Policy