feat: serve width-bounded thumbnail variants for image grids

Add /files/{key}?w=<px>: the endpoint decodes JPEG/PNG sources, downscales to
a width snapped to a small allow-list (aspect-preserving, never upscaling),
caches the result under a thumbs/ prefix, and serves it — so cover grids
download ~KB instead of the 1–5 MB original. Cover handlers purge cached
variants when a cover (whose key is reused) is replaced or deleted. Frontend
grids use new thumbUrl/thumbSrcset helpers (MangaCard with responsive srcset).

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.124.24",
+  "version": "0.125.0",
   "private": true,
   "type": "module",
   "scripts": {