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/utils/dom.ts

@@ -180,6 +180,11 @@ export type RendererDom = {
   changelogStatus: HTMLDivElement;
   changelogList: HTMLDivElement;
 
+  pluginPanelModal: HTMLDivElement;
+  pluginPanelTitle: HTMLDivElement;
+  pluginPanelClose: HTMLButtonElement;
+  pluginPanelBody: HTMLDivElement;
+
   sessionHelpModal: HTMLDivElement;
   sessionHelpClose: HTMLButtonElement;
   sessionHelpShortcut: HTMLDivElement;
@@ -448,6 +453,10 @@ export function resolveRendererDom(): RendererDom {
     changelogWarning: getRequiredElement<HTMLDivElement>('changelogWarning'),
     changelogStatus: getRequiredElement<HTMLDivElement>('changelogStatus'),
     changelogList: getRequiredElement<HTMLDivElement>('changelogList'),
+    pluginPanelModal: getRequiredElement<HTMLDivElement>('pluginPanelModal'),
+    pluginPanelTitle: getRequiredElement<HTMLDivElement>('pluginPanelTitle'),
+    pluginPanelClose: getRequiredElement<HTMLButtonElement>('pluginPanelClose'),
+    pluginPanelBody: getRequiredElement<HTMLDivElement>('pluginPanelBody'),
     sessionHelpModal: getRequiredElement<HTMLDivElement>('sessionHelpModal'),
     sessionHelpClose: getRequiredElement<HTMLButtonElement>('sessionHelpClose'),
     sessionHelpShortcut: getRequiredElement<HTMLDivElement>('sessionHelpShortcut'),