feat: playback overlay with seek bar, auto-hide and double-tap skip (v0.3.4)

New PlaybackOverlay component replaces the old hint bar in ChannelScreen.
The overlay sits as a fullscreen layer above the app during playback:

- Auto-hides after 5 s while playing; stays visible when paused;
  reactivated by any gamepad key press or mouse movement
- Center play/pause button, channel avatar, title, LIVE badge
- VOD: seek bar with pointer-capture scrubbing, current time / total duration
- Live: pulsing LIVE indicator instead of seek bar
- Double-tap zones left/right (±10 s) with animated skip feedback
- Gamepad hints in the top bar
- New icons: PlayIcon, PauseIcon, StopIcon in Icons.tsx
- New IPC channel `playback:time-update` (~1 Hz) pushes absolute playback
  position from main process to renderer; adds PlaybackTimeUpdate type
  and onTimeUpdate bridge in preload + t4sd.d.ts
- PlaybackEvent extended with durationSeconds and isLive fields

src/preload/index.ts

@@ -29,6 +29,7 @@ const IPC = {
   playbackSetLoggingEnabled: 'playback:set-logging-enabled',
   playbackGetLogPath: 'playback:get-log-path',
   playbackEvent: 'playback:event',
+  playbackTimeUpdate: 'playback:time-update',
 
   gamepadInput: 'gamepad-input'
 } as const
@@ -74,6 +75,8 @@ export interface PlaybackEvent {
   kind: 'started' | 'stopped' | 'error'
   channelLogin?: string
   message?: string
+  durationSeconds?: number
+  isLive?: boolean
 }
 
 export interface GameInfo {
@@ -163,6 +166,11 @@ const api = {
       const listener = (_e: IpcRendererEvent, event: PlaybackEvent): void => cb(event)
       ipcRenderer.on(IPC.playbackEvent, listener)
       return () => ipcRenderer.removeListener(IPC.playbackEvent, listener)
+    },
+    onTimeUpdate: (cb: (data: { positionSeconds: number }) => void): (() => void) => {
+      const listener = (_e: IpcRendererEvent, data: { positionSeconds: number }): void => cb(data)
+      ipcRenderer.on(IPC.playbackTimeUpdate, listener)
+      return () => ipcRenderer.removeListener(IPC.playbackTimeUpdate, listener)
     }
   },
   gamepad: {