fix(web): referrer-policy same-origin so same-origin POSTs keep a real Origin

no-referrer downgraded the Origin header to the literal "null" on same-origin non-GET requests (per the WHATWG Fetch "Append a request Origin header" algorithm), which the /api/* csrfOriginCheck then rejected with 403 "bad origin" — silently breaking QR login, cookie save, and every other WebUI POST/PUT/DELETE/PATCH (playback, bot management, user admin). /api/session/* was unaffected because it mounts before the CSRF gate, which is why WebUI login still worked.

same-origin keeps the real Origin on same-origin requests (CSRF passes) while still sending no Referer cross-origin, so B站/NetEase/QQ CDN cover thumbnails keep loading. Adds referrer-policy.test.ts pinning the policy and a csrf.test.ts case for the Origin: "null" rejection.

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

web/index.html

@@ -3,10 +3,18 @@
 <head>
   <meta charset="UTF-8">
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
-  <!-- Bilibili / NetEase / QQ image CDNs reject requests whose Referer is not on their whitelist.
-       Setting no-referrer at the document level covers <img> tags AND CSS background-image fetches.
-       Our own /api/* CSRF check uses Origin (not Referer), so this doesn't break auth. -->
-  <meta name="referrer" content="no-referrer">
+  <!-- Bilibili / NetEase / QQ image CDNs reject requests whose Referer is not on
+       their whitelist, so we must not leak a Referer cross-origin. "same-origin"
+       does exactly that: full Referer for our own requests, none for cross-origin
+       ones — so cover thumbnails (<img> AND CSS background-image) still load.
+       Do NOT switch this back to "no-referrer": per the WHATWG Fetch spec
+       ("Append a request Origin header") no-referrer downgrades the Origin header
+       to the literal string "null" on same-origin non-GET requests. The /api/*
+       CSRF guard (src/web/middleware/csrf.ts) then can't parse a host from it and
+       responds 403 "bad origin", silently breaking EVERY POST/PUT/DELETE — QR
+       login, cookie save, playback controls, bot management, user admin, etc.
+       "same-origin" keeps the real Origin on same-origin requests, so CSRF passes. -->
+  <meta name="referrer" content="same-origin">
   <title>TSMusicBot</title>
   <link rel="preconnect" href="https://fonts.googleapis.com">
   <link href="https://fonts.googleapis.com/css2?family=Barlow:wght@400;500;600;700;800&display=swap" rel="stylesheet">