fix: cache immutable blobs served from /files

Page and cover blobs are content-addressed by unguessable, immutable keys,
but /files sent no Cache-Control, so the browser refetched on every reader
open — defeating the whole-chapter and next-chapter preloading (and re-
proxying every byte through the node server in prod). Send
`Cache-Control: public, max-age=31536000, immutable`.

Bump to 0.124.3.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.124.2",
+  "version": "0.124.3",
   "private": true,
   "type": "module",
   "scripts": {