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/PlayerControls.svelte

@@ -115,11 +115,9 @@
     }, 100);
   }
 
-  function handleFullscreen() {
-    toggleFullscreen();
-    setTimeout(() => {
-      fullscreenActive = isFullscreenActive();
-    }, 100);
+  async function handleFullscreen() {
+    await toggleFullscreen();
+    fullscreenActive = await isFullscreenActive();
   }
 
   function handleCCContextMenu(e: MouseEvent) {
@@ -129,9 +127,9 @@
 
   // Sync PiP/fullscreen/rate state periodically
   $effect(() => {
-    const interval = setInterval(() => {
+    const interval = setInterval(async () => {
       pipActive = isPiPActive();
-      fullscreenActive = isFullscreenActive();
+      fullscreenActive = await isFullscreenActive();
       currentRate = getPlaybackRate();
     }, 500);
     return () => clearInterval(interval);
@@ -279,7 +277,7 @@
 <style>
   .player-controls-overlay {
     position: absolute;
-    bottom: 24px;
+    bottom: 48px;
     left: 15%;
     right: 15%;
     pointer-events: none;