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/lib/theme/applyMetaThemeColor.svelte.ts

@@ -1,5 +1,8 @@
 import { resolvedTheme } from '$lib/stores/theme.svelte';
 
+// TODO(#375): keep these hex values in sync with tokens.json
+// colors.dark.obsidian / colors.light.obsidian. The duplication is a known
+// drift hazard from #363; consume from the generated CSS layer instead.
 const DARK = '#14171A';
 const LIGHT = '#F8F5EE';