chore(web): post-#363 cleanup — DRY tripwires, drop unused description, singular playlist title

M2 — Add TODO(#375) tripwire comments where the dark/light theme-color
hex pair is duplicated outside tokens.json (vite.config.ts and
applyMetaThemeColor.svelte.ts). Refactoring is out of scope for #363;
the comments are stop signs for the next person who edits these.

M5 — The SPA never reads window.__MINSTREL__.description. The OG meta
description is server-rendered and complete on its own. Drop the dead
inline-script field and the corresponding helpers in branding.ts.
Server-side BrandingConfig.Description stays — it's still used for
<meta name="description"> and og:description.

Copy nit — Playlist detail page title becomes "Playlist · Foo"
matching the singular form already used by Artist · / Album ·.

web/src/routes/playlists/[id]/+page.svelte

@@ -138,7 +138,7 @@
 </script>
 
 <svelte:head>
-  <title>{pageTitle(playlistQuery?.data?.name ? `Playlists · ${playlistQuery.data.name}` : 'Playlists')}</title>
+  <title>{pageTitle(playlistQuery?.data?.name ? `Playlist · ${playlistQuery.data.name}` : 'Playlist')}</title>
 </svelte:head>
 
 <div class="mx-auto max-w-4xl px-4 py-6">