fix: force download for untyped (octet-stream) served blobs

The /files handler served the application/octet-stream fallback with no
Content-Disposition, letting a browser render it inline — a stored-XSS vector if
the blob is crafted HTML/JS. Add Content-Disposition: attachment for that type
(belt to the existing nosniff); known image types stay inline.

Test (new tests/api_files.rs): octet-stream → attachment; png → inline.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.128.8",
+  "version": "0.128.9",
   "private": true,
   "type": "module",
   "scripts": {