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/handlers/keyboard.ts

@@ -32,6 +32,7 @@ export function createKeyboardHandlers(
     handleControllerDebugKeydown: (e: KeyboardEvent) => boolean;
     handleSessionHelpKeydown: (e: KeyboardEvent) => boolean;
     handleChangelogKeydown: (e: KeyboardEvent) => boolean;
+    handlePluginPanelKeydown: (e: KeyboardEvent) => boolean;
     openSessionHelpModal: (opening: {
       bindingKey: 'KeyH' | 'KeyK';
       fallbackUsed: boolean;
@@ -1172,6 +1173,11 @@ export function createKeyboardHandlers(
         return;
       }
 
+      if (ctx.state.pluginPanelModalOpen) {
+        options.handlePluginPanelKeydown(e);
+        return;
+      }
+
       if (ctx.state.runtimeOptionsModalOpen) {
         options.handleRuntimeOptionsKeydown(e);
         return;