fix: QA round 2 — playhead guard, controls position, caption lift, Tauri fullscreen, timeline sync

- Guard drawPlayhead on session.duration > 0 in PlayerTimeline
- Move floating controls pill from bottom: 24px to 48px
- Add playerControlsVisible prop to VideoPlayer; captions slide up 160px when controls visible
- Replace web Fullscreen API with Tauri getCurrentWindow().setFullscreen() (WKWebView compat)
- Add core:window:allow-set-fullscreen and core:window:allow-is-fullscreen permissions
- Tie PlayerTimeline visibility to showPlayerControls; remove unused proximity logic

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/lib/transport/playback.test.ts

@@ -11,6 +11,15 @@ vi.mock('$lib/stores/videoSession.svelte', () => ({
   },
 }));
 
+// Mock Tauri window API for fullscreen
+const mockTauriWindow = {
+  isFullscreen: vi.fn().mockResolvedValue(false),
+  setFullscreen: vi.fn().mockResolvedValue(undefined),
+};
+vi.mock('@tauri-apps/api/window', () => ({
+  getCurrentWindow: () => mockTauriWindow,
+}));
+
 import {
   setVideoElement,
   togglePiP,
@@ -70,36 +79,30 @@ describe('playback — PiP helpers', () => {
 });
 
 describe('playback — fullscreen helpers', () => {
-  let mockVideo: HTMLVideoElement;
-
   beforeEach(() => {
-    mockVideo = createMockVideoElement();
-    setVideoElement(mockVideo);
-    Object.defineProperty(document, 'fullscreenElement', {
-      value: null,
-      writable: true,
-      configurable: true,
-    });
+    mockTauriWindow.isFullscreen.mockResolvedValue(false);
+    mockTauriWindow.setFullscreen.mockResolvedValue(undefined);
   });
 
-  it('isFullscreenActive returns false when not fullscreen', () => {
-    expect(isFullscreenActive()).toBe(false);
+  it('isFullscreenActive returns false when not fullscreen', async () => {
+    expect(await isFullscreenActive()).toBe(false);
   });
 
-  it('toggleFullscreen calls requestFullscreen on document.documentElement', () => {
-    const spy = vi.fn().mockResolvedValue(undefined);
-    document.documentElement.requestFullscreen = spy;
-    toggleFullscreen();
-    expect(spy).toHaveBeenCalled();
+  it('toggleFullscreen calls setFullscreen(true) when not fullscreen', async () => {
+    mockTauriWindow.isFullscreen.mockResolvedValue(false);
+    await toggleFullscreen();
+    expect(mockTauriWindow.setFullscreen).toHaveBeenCalledWith(true);
   });
 
-  it('isFullscreenActive returns true when fullscreen element exists', () => {
-    Object.defineProperty(document, 'fullscreenElement', {
-      value: mockVideo,
-      writable: true,
-      configurable: true,
-    });
-    expect(isFullscreenActive()).toBe(true);
+  it('toggleFullscreen calls setFullscreen(false) when fullscreen', async () => {
+    mockTauriWindow.isFullscreen.mockResolvedValue(true);
+    await toggleFullscreen();
+    expect(mockTauriWindow.setFullscreen).toHaveBeenCalledWith(false);
+  });
+
+  it('isFullscreenActive returns true when fullscreen', async () => {
+    mockTauriWindow.isFullscreen.mockResolvedValue(true);
+    expect(await isFullscreenActive()).toBe(true);
   });
 });