fix(crawler): send a realistic browser User-Agent (Cloudflare evasion)

The crawler's headless Chromium advertised the default HeadlessChrome UA,
which Cloudflare's bot detection challenges on sight — over Tor that meant an
unsolvable 'Just a moment' page on every fetch, so session verification kept
classifying pages as transient and the metadata pass failed at
'acquire browser lease'. (Confirmed empirically: same Tor exit + a normal
Chrome UA returns the real page with the #logo sentinel; the HeadlessChrome
UA returns the challenge.) The existing user_agent setting only fed reqwest,
never the browser. Override the launcher's UA with a realistic non-headless
default (CRAWLER_USER_AGENT to customize); the UA may contain spaces, unlike
CRAWLER_BROWSER_ARGS.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.81.1",
+  "version": "0.81.2",
   "private": true,
   "type": "module",
   "scripts": {