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>

src/api.js

@@ -194,6 +194,46 @@ export async function removeRomajiOverride(kanji) {
   } catch (e) { console.warn('removeRomajiOverride failed:', e); }
 }
 
+// ─── ARTIST OVERRIDES (pin a local artist → a specific Deezer artist; shared) ───
+// Fixes same-name collisions on the artist page (fans / top songs / albums / image).
+let _artistOverridesCache = null;
+export function getArtistOverrides() { return _artistOverridesCache || {}; }
+export function getArtistOverride(ndArtistId) { return (_artistOverridesCache || {})[String(ndArtistId)] || null; }
+
+export async function loadArtistOverrides() {
+  if (!S.cfg?.backendUrl) { _artistOverridesCache = {}; return; }
+  try {
+    const r = await fetch(`${S.cfg.backendUrl}/api/artist-overrides`);
+    _artistOverridesCache = r.ok ? await r.json() : {};
+  } catch { _artistOverridesCache = {}; }
+}
+
+export async function setArtistOverride(ndArtistId, deezer) {
+  if (!S.cfg?.backendUrl || !ndArtistId || !deezer?.deezerArtistId) return;
+  const entry = {
+    deezerArtistId: String(deezer.deezerArtistId),
+    name:  deezer.name  || '',
+    image: deezer.image || '',
+    fanCount: deezer.fanCount ?? null,
+  };
+  try {
+    await fetch(`${S.cfg.backendUrl}/api/artist-overrides`, {
+      method: 'POST', headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ ndArtistId: String(ndArtistId), ...entry }),
+    });
+    if (!_artistOverridesCache) _artistOverridesCache = {};
+    _artistOverridesCache[String(ndArtistId)] = entry;
+  } catch (e) { console.warn('setArtistOverride failed:', e); }
+}
+
+export async function removeArtistOverride(ndArtistId) {
+  if (!S.cfg?.backendUrl || !ndArtistId) return;
+  try {
+    await fetch(`${S.cfg.backendUrl}/api/artist-overrides/${encodeURIComponent(ndArtistId)}`, { method: 'DELETE' });
+    if (_artistOverridesCache) delete _artistOverridesCache[String(ndArtistId)];
+  } catch (e) { console.warn('removeArtistOverride failed:', e); }
+}
+
 export function dispArtistById(id, name) {
   if (!S.cfg.showRomaji) return name || '';
   const ov = getRomajiOverrides();