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/core/services/ipc-command.test.ts

@@ -16,6 +16,7 @@ function createOptions(overrides: Partial<Parameters<typeof handleMpvCommandFrom
       SHIFT_SUB_DELAY_TO_NEXT_SUBTITLE_START: '__sub-delay-next-line',
       SHIFT_SUB_DELAY_TO_PREVIOUS_SUBTITLE_START: '__sub-delay-prev-line',
       YOUTUBE_PICKER_OPEN: '__youtube-picker-open',
+      PLAYLIST_BROWSER_OPEN: '__playlist-browser-open',
     },
     triggerSubsyncFromConfig: () => {
       calls.push('subsync');
@@ -26,6 +27,9 @@ function createOptions(overrides: Partial<Parameters<typeof handleMpvCommandFrom
     openYoutubeTrackPicker: () => {
       calls.push('youtube-picker');
     },
+    openPlaylistBrowser: () => {
+      calls.push('playlist-browser');
+    },
     runtimeOptionsCycle: () => ({ ok: true }),
     showMpvOsd: (text) => {
       osd.push(text);
@@ -110,6 +114,14 @@ test('handleMpvCommandFromIpc dispatches special youtube picker open command', (
   assert.deepEqual(osd, []);
 });
 
+test('handleMpvCommandFromIpc dispatches special playlist browser open command', () => {
+  const { options, calls, sentCommands, osd } = createOptions();
+  handleMpvCommandFromIpc(['__playlist-browser-open'], options);
+  assert.deepEqual(calls, ['playlist-browser']);
+  assert.deepEqual(sentCommands, []);
+  assert.deepEqual(osd, []);
+});
+
 test('handleMpvCommandFromIpc does not forward commands while disconnected', () => {
   const { options, sentCommands, osd } = createOptions({
     isMpvConnected: () => false,