fix(analysis-vision): pin spawn_blocking dispatch + warn on undecodable fallback (0.87.14)

Two 0.87.5 follow-ups:

1. Pin spawn_blocking dispatch: new current_thread runtime test runs
   analyze() on a 6 MP image while a counter task ticks every 5 ms.
   Without spawn_blocking the counter is starved (0 ticks); with it
   the runtime stays responsive (≥2 ticks). Mutation-confirmed.

2. Warn on Undecodable fallback: five fallback paths in prepare_analysis
   now emit a tracing::warn! with dimensions so a JPEG encoder
   regression isn't indistinguishable from "the page was just garbage."

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.87.13",
+  "version": "0.87.14",
   "private": true,
   "type": "module",
   "scripts": {