feat(artist): pin a local artist to a specific Deezer artist (same-name fix, shared)

Fixes same-name collisions where NovaSonic guessed the wrong Deezer artist (e.g.
'Nana' → a random DJ): fans, top songs, missing albums and the hero image were all
off. A '⚙ Falscher Künstler?' button on the artist page opens a picker (same-name
Deezer candidates with image + fan count); the chosen mapping is stored SERVER-SIDE
(shared across users, like the romaji overrides) in /data/deezer-artist-overrides.json.

Once pinned, the artist page short-circuits the name search to the pinned Deezer id
(drives albums/top-songs/fans) and overrides the hero image with the pinned artist's
Deezer picture (NovaSonic-side, so it works even for tag-merged same-name artists —
Navidrome can't disambiguate them without an MBID). New endpoints
GET/POST/DELETE /api/artist-overrides; api.js load/get/set/remove; loaded at startup.
SW cache -> ns-v74. Needs on-device runtime test.

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

backend/novasonic-server.js

@@ -233,6 +233,47 @@ app.delete('/api/romaji-overrides/:kanji', (req, res) => {
   res.json({ ok: true });
 });
 
+// ─── ARTIST OVERRIDES (pin a local artist → a specific Deezer artist; shared) ──
+// Fixes same-name collisions where NovaSonic would otherwise guess the wrong Deezer artist
+// (fans / top songs / albums / image). Keyed by the Navidrome artist id.
+const _artistOverridesFile = path.join(CONFIG.dataPath, 'deezer-artist-overrides.json');
+function _loadArtistOverrides() {
+  try { return JSON.parse(fs.readFileSync(_artistOverridesFile, 'utf8')); } catch { return {}; }
+}
+function _saveArtistOverrides(map) {
+  try {
+    fs.mkdirSync(CONFIG.dataPath, { recursive: true });
+    fs.writeFileSync(_artistOverridesFile, JSON.stringify(map, null, 2), 'utf8');
+  } catch (e) { console.error('Failed to save artist overrides:', e.message); }
+}
+
+// GET — full map { ndArtistId: { deezerArtistId, name, image, fanCount } }
+app.get('/api/artist-overrides', (_req, res) => res.json(_loadArtistOverrides()));
+
+// POST — upsert a pin { ndArtistId, deezerArtistId, name, image, fanCount }
+app.post('/api/artist-overrides', (req, res) => {
+  const { ndArtistId, deezerArtistId, name, image, fanCount } = req.body || {};
+  if (!ndArtistId || !deezerArtistId) return res.status(400).json({ error: 'ndArtistId and deezerArtistId required' });
+  const map = _loadArtistOverrides();
+  map[String(ndArtistId)] = {
+    deezerArtistId: String(deezerArtistId),
+    name:  name  || '',
+    image: image || '',
+    fanCount: (fanCount ?? null),
+  };
+  _saveArtistOverrides(map);
+  res.json({ ok: true });
+});
+
+// DELETE /api/artist-overrides/:ndArtistId — remove a pin
+app.delete('/api/artist-overrides/:ndArtistId', (req, res) => {
+  const id = decodeURIComponent(req.params.ndArtistId);
+  const map = _loadArtistOverrides();
+  delete map[id];
+  _saveArtistOverrides(map);
+  res.json({ ok: true });
+});
+
 // POST /api/romaji-convert — convert an array of strings to romaji
 app.post('/api/romaji-convert', (req, res) => {
   const { texts } = req.body;