feat(scrobble stats): genre ranking (Top-Genres) — web + android

Play-log now stores each play's genre (from the song tags; web scrobbleLog +
android PlayLogClient capture song.genre). The stats endpoint returns topGenres
(GROUP BY lower(genre), case-insensitive, blanks ignored); web + android render a
Top-Genres section. Schema migrates via ALTER TABLE. Old plays have no genre
(fills going forward, like cover art). SW cache -> ns-v72.

Android compileDebugKotlin + web build green; genre grouping + null-ignore unit-tested.

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

src/player/scrobbleLog.js

@@ -41,6 +41,7 @@ export function logPlay(song, source = 'web') {
     album:    song.album    ?? null,
     albumId:  song.albumId  ?? null,
     coverArt: song.coverArt ?? null,   // Subsonic coverArt token — what getCoverArt actually needs
+    genre:    song.genre    ?? null,
     duration: Number.isFinite(Number(song.duration)) ? Math.floor(Number(song.duration)) : null,
     playedAt: Math.floor(Date.now() / 1000),   // Unix seconds, UTC
     source,