fix: use the US iTunes store everywhere for consistent artist naming

Search (web api.js) was JP-first, the download lookups were JP-first, and the Android
client was already US — so the same artist resolved inconsistently: sometimes kanji,
sometimes katakana, sometimes romaji, depending on which path and which store matched.

Switch everything to the US store:
- api.js itunesSearch → country=us (one fetch, matches the Android client)
- metadata.js lookupItunesTrackMeta → US only (drops the JP fetch: consistent names +
  halves the download's iTunes calls, which also eases the rate limit)
- searchItunesAlbumTracks → country=us
- remove the JP-katakana guards in the album/artist download paths (redundant now, and
  they wrongly romanized genuine Japanese artists like 羽岡佳 → "Kei Haneoka")

Titles still come from the source, so a Japanese title the user picked stays Japanese;
only iTunes-derived fields (album_artist, genre, year, artwork) follow the US store.

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

public/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'ns-v46';
+const CACHE = 'ns-v47';
 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.