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-tauri/capabilities/default.json

@@ -13,6 +13,8 @@
     "store:allow-set",
     "store:allow-save",
     "store:allow-load",
-    "process:allow-exit"
+    "process:allow-exit",
+    "core:window:allow-set-fullscreen",
+    "core:window:allow-is-fullscreen"
   ]
 }