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>

src/lib/components/PlayerTimeline.svelte

@@ -56,7 +56,9 @@
     }
 
     // Draw playhead
-    drawPlayhead(ctx, timelineState, session.currentTime);
+    if (session.duration > 0) {
+      drawPlayhead(ctx, timelineState, session.currentTime);
+    }
 
     ctx.restore();
   }