Refactor Jellyfin plugin: integrate UI components and implement backend contract.

- Integrated "Recommend Film" button and Rating UI into Jellyfin web interface via MutationObserver.
- Implemented full library sync (metadata + user state) in MovieNightSyncService.
- Updated MovieNightBackendClient to support sync, recommendations, ratings, and viewed status.
- Added proxy endpoints to MovieNightController for frontend-backend communication.
- Refined sync logic to handle large libraries and ensured .NET 9.0 compatibility.

Co-authored-by: devitq <118541411+devitq@users.noreply.github.com>

plugins/jellyfin/Jellyfin.Plugin.MovieNight/Configuration/configPage.html

@@ -53,6 +53,12 @@
                             <span>Test connection</span>
                         </button>
                     </div>
+
+                    <div style="margin-top: 2em; padding: 1em; background: #333; border-radius: 4px;">
+                        <h3>UI Integration</h3>
+                        <p>To enable the "Recommend Film" button and Rating UI in the main Jellyfin interface, you must add the following script URL to your Jellyfin <strong>Custom JavaScript</strong> setting (Dashboard > General):</p>
+                        <code id="UIScriptUrl" style="display: block; padding: 0.5em; background: #000; word-break: break-all;"></code>
+                    </div>
                 </form>
             </div>
         </div>