fix(web): set no-referrer at document level so B站 cover thumbnails load

Bilibili's CDN (i*.hdslb.com) returns 403 with `x-error-info:
RefererWhite` for image requests whose Referer is not on their
whitelist. `CoverArt.vue` already sets `referrerpolicy="no-referrer"`
on its `<img>` tag, BUT the `.cover-shadow` div renders the same URL
as a CSS `background-image`, which ignores the img attribute and uses
the document default policy (`strict-origin-when-cross-origin` in
modern Firefox/Chrome) — that sends `Referer: http://localhost:3000/`
and triggers the block.

Setting `<meta name="referrer" content="no-referrer">` in index.html
applies no-referrer site-wide: covers <img> tags, CSS background-image
fetches, and anywhere else CDNs check referer. Doesn't affect our
/api/* CSRF middleware because that uses Origin (still sent by the
browser), not Referer.

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

web/index.html

@@ -3,6 +3,10 @@
 <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">
   <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">