Refactor monolithic app.js into 8 modular files

Split 3803-line app.js into focused modules:
- core.js: shared state, utilities, i18n, API commands, MDI icons
- player.js: tabs, theme, accent, vinyl, visualizer, UI updates
- websocket.js: connection, auth, reconnection
- scripts.js: scripts CRUD, quick access, execution dialog
- callbacks.js: callbacks CRUD
- browser.js: media file browser, thumbnails, pagination, search
- links.js: links CRUD, header links, display controls
- main.js: DOMContentLoaded init orchestrator

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

media_server/static/index.html

@@ -638,6 +638,13 @@
         </div>
     </footer>
 
-    <script src="/static/js/app.js"></script>
+    <script src="/static/js/core.js"></script>
+    <script src="/static/js/player.js"></script>
+    <script src="/static/js/websocket.js"></script>
+    <script src="/static/js/scripts.js"></script>
+    <script src="/static/js/callbacks.js"></script>
+    <script src="/static/js/browser.js"></script>
+    <script src="/static/js/links.js"></script>
+    <script src="/static/js/main.js"></script>
 </body>
 </html>