Codebase audit fixes: stability, performance, accessibility

- Fix CORS: set allow_credentials=False (token auth, not cookies)
- Add threading.Lock for position cache thread safety
- Add shutdown_executor() for clean ThreadPoolExecutor cleanup
- Dedicated ThreadPoolExecutors for script/callback execution
- Fix Mutagen file handle leaks with try/finally close
- Reduce idle WebSocket polling (0.5s → 2.0s when no clients)
- Add :focus-visible styles for playback control buttons
- Add aria-label to icon-only header buttons
- Dynamic album art alt text for screen readers
- Persist MDI icon cache to localStorage

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

media_server/static/js/app.js

@@ -1368,6 +1368,13 @@
             currentState = status.state;
             updatePlaybackState(status.state);
 
+            // Update album art alt text for accessibility
+            const altText = status.title && status.artist
+                ? `${status.artist} – ${status.title}`
+                : status.title || t('player.no_media');
+            dom.albumArt.alt = altText;
+            dom.miniAlbumArt.alt = altText;
+
             // Update album art (skip if same track to avoid redundant network requests)
             const artworkSource = status.album_art_url || null;
             const artworkKey = `${status.title || ''}|${status.artist || ''}|${artworkSource || ''}`;
@@ -3468,7 +3475,16 @@ async function toggleDisplayPower(monitorId, monitorName) {
 // Header Quick Links
 // ============================================================
 
-const mdiIconCache = {};
+// In-memory + localStorage cache for MDI icons (persists across reloads)
+const mdiIconCache = (() => {
+    try {
+        return JSON.parse(localStorage.getItem('mdiIconCache') || '{}');
+    } catch { return {}; }
+})();
+
+function _persistMdiCache() {
+    try { localStorage.setItem('mdiIconCache', JSON.stringify(mdiIconCache)); } catch {}
+}
 
 async function fetchMdiIcon(iconName) {
     // Parse "mdi:icon-name" → "icon-name"
@@ -3480,6 +3496,7 @@ async function fetchMdiIcon(iconName) {
         if (response.ok) {
             const svg = await response.text();
             mdiIconCache[name] = svg;
+            _persistMdiCache();
             return svg;
         }
     } catch (e) {