feat(plugins): add JavaScript plugin system with overlay panel

- Load plugins from <config dir>/plugins/<id>/ when enabled under `plugins` in config
- Expose ctx with mpv events/commands, timers, subtitle context, actions, notify, exec, and panel
- Bind plugin actions via `__plugin-action:<id>/<action>` keybindings (runPluginAction session action)
- Add plugin-panel modal with markdown rendering; closing aborts the panel signal
- Add ai-explain example plugin that queries `codex exec` for the current line
- Add e2e harness `files` option and plugin e2e scenario; document plugins in docs-site and internal docs

src/renderer/index.html

@@ -1019,6 +1019,15 @@
           </div>
         </div>
       </div>
+      <div id="pluginPanelModal" class="modal hidden" aria-hidden="true">
+        <div class="modal-content plugin-panel-content">
+          <div class="modal-header">
+            <div id="pluginPanelTitle" class="modal-title"></div>
+            <button id="pluginPanelClose" class="modal-close" type="button">Close</button>
+          </div>
+          <div id="pluginPanelBody" class="modal-body plugin-panel-body" tabindex="0"></div>
+        </div>
+      </div>
       <div id="playlistBrowserModal" class="modal hidden" aria-hidden="true">
         <div class="modal-content playlist-browser-content">
           <div class="modal-header">