feat(scrobble): Head-to-Head "Musik-Match" — compare two users' listening

Adds GET /scrobble/users (distinct users for the partner picker) and
GET /scrobble/compare?with= to the scrobble service. Taste match = cosine
of the two users' artist-play vectors (0..100); also returns shared artists
(mirrored bars), shared tracks, and each side's unique top artists. Reuses
the existing per-user play-log — no new data capture.

Web UI src/ui/compare.js (overlay from Settings "🎧 Musik-Match"): partner
picker fed by /scrobble/users, match-score ring, per-user totals, shared +
unique lists with clickable artist/album rows. Cosine + shared logic verified
against a hand computation on an in-memory DB. Android pending (parity: 🔜).

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

public/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'ns-v76';
+const CACHE = 'ns-v77';
 const PRECACHE = ['/', 'index.html', 'manifest.json', 'icon.svg'];
 const SKIP_PATHS = ['/rest/stream', '/rest/download', '/config.js'];
 // Auth token params stripped from cache keys so artwork survives across sessions/token refreshes.