feat: add playlist browser overlay modal

- Add overlay modal for browsing sibling video files and live mpv queue
- Add IPC commands for playlist operations (add, remove, move, play)
- Add playlist-browser-runtime and playlist-browser-sort modules
- Add keyboard handler and preload bindings for playlist browser
- Add default Ctrl+Alt+P keybinding to open the modal
- Add HTML structure, renderer wiring, and state for the modal
- Add changelog fragment and docs updates

src/main/runtime/ipc-bridge-actions-main-deps.test.ts

@@ -14,6 +14,7 @@ test('ipc bridge action main deps builders map callbacks', async () => {
       triggerSubsyncFromConfig: async () => {},
       openRuntimeOptionsPalette: () => {},
       openYoutubeTrackPicker: () => {},
+      openPlaylistBrowser: () => {},
       cycleRuntimeOption: () => ({ ok: false as const, error: 'x' }),
       showMpvOsd: () => {},
       replayCurrentSubtitle: () => {},