feat: add Player/Clipper mode switching with auto-hide UI

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

src/app.css

@@ -37,6 +37,11 @@ body {
   height: 100vh;
 }
 
+/* Player mode: app shell becomes relative for overlay positioning */
+.app-shell.player-mode {
+  position: relative;
+}
+
 button {
   cursor: pointer;
   border: none;