fix: playlist matching for romaji-stored Japanese tracks + punctuation cleanup

- Clean Japanese punctuation (、。・) in romajiForStorage so it doesn't pass through
- Add POST /api/romaji-convert endpoint for batch title conversion
- Add romajiConvert() to api.js
- Add third fallback in _createNdPlaylist: convert Japanese title to romaji and
  search Navidrome with it (for songs already downloaded with romaji tags)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/api.js

@@ -68,6 +68,20 @@ export async function fetchRomajiIndex() {
 
 export function invalidateRomajiIndex() { _romajiIndex = null; }
 
+export async function romajiConvert(texts) {
+  if (!S.cfg.backendUrl || !texts.length) return texts;
+  try {
+    const r = await fetch(`${S.cfg.backendUrl}/api/romaji-convert`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ texts }),
+    });
+    if (!r.ok) return texts;
+    const { results } = await r.json();
+    return results;
+  } catch { return texts; }
+}
+
 // ─── ITUNES ───
 // Route through backend proxy to avoid CORS; fall back to direct if no backend configured.
 function _itunesBase() {