fix(analysis): cap decoded pixels on the vision backend too

The vision prep pass decoded pages with bare `image::load_from_memory`,
which applies no allocation bound. `max_pixels` only downscales after a
full decode, so a tiny WebP/JPEG header declaring huge dimensions could
OOM the blocking worker — the same decompression-bomb the OCR backend
already guards against. Manga pages are commonly WebP/JPEG, where the
format's own self-limits are weaker than PNG's.

Route the decode through `decode_within`, applying an `image::Limits`
alloc cap sized from the shared `ocr_max_decode_pixels`
(ANALYSIS_OCR_MAX_DECODE_PIXELS). Add real-WebP bomb coverage asserting
both the helper and the end-to-end Undecodable fallback.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.93.6",
+  "version": "0.93.7",
   "private": true,
   "type": "module",
   "scripts": {