feat(windows): taskbar thumbnail toolbar with GDI media icons

Implements ITaskbarList3::ThumbBarAddButtons for Prev/Play-Pause/Next
buttons in the Windows taskbar thumbnail preview. Icons are drawn at
runtime via GDI (no binary assets). WndProc subclass intercepts
THBN_CLICKED and emits the same media:* events as souvlaki/tray.
update_taskbar_icon command swaps Play↔Pause icon on state change.
Frontend syncs via playerStore subscribe alongside mpris_set_playback.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/store/playerStore.ts

@@ -543,6 +543,7 @@ export function initAudioListeners(): () => void {
         playing: isPlaying,
         positionSecs: currentTime > 0 ? currentTime : null,
       }).catch(() => {});
+      invoke('update_taskbar_icon', { isPlaying }).catch(() => {});
       return;
     }