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>

src/api.js

@@ -275,12 +275,10 @@ function _itunesBase() {
 
 export async function itunesSearch(query, entity='musicTrack', limit=25) {
   try {
-    const base = `${_itunesBase()}/search?term=${encodeURIComponent(query)}&media=music&entity=${entity}&limit=${limit}&explicit=Yes`;
-    const rJp = await fetch(`${base}&country=jp`);
-    const jpResults = JSON.parse(await rJp.text()).results || [];
-    if (jpResults.length) return jpResults;
-    const rUs = await fetch(`${base}&country=us`);
-    return JSON.parse(await rUs.text()).results || [];
+    // US store only — one consistent naming source across search + download (matches the Android
+    // client, which is already US). A single fetch also halves the iTunes request rate.
+    const r = await fetch(`${_itunesBase()}/search?term=${encodeURIComponent(query)}&media=music&entity=${entity}&limit=${limit}&country=us&explicit=Yes`);
+    return JSON.parse(await r.text()).results || [];
   } catch { return []; }
 }