feat(studio): fullscreen preview mode (F key)

Add distraction-free fullscreen mode using the HTML5 Fullscreen API.
Press F to enter fullscreen (Esc to exit). When active, the composition
fills the screen — timeline, sidebars, and editing overlays are hidden
while all playback shortcuts (Space, J/K/L, arrows, etc.) remain
functional. A fullscreen toggle button is also added to player controls.

Closes #995

packages/studio/src/hooks/useAppHotkeys.ts

@@ -248,6 +248,24 @@ export function useAppHotkeys({
       }
     }
 
+    // F — toggle fullscreen preview
+    if (
+      event.key.toLowerCase() === "f" &&
+      !event.metaKey &&
+      !event.ctrlKey &&
+      !event.altKey &&
+      !event.shiftKey &&
+      !isEditableTarget(event.target)
+    ) {
+      event.preventDefault();
+      if (document.fullscreenElement) {
+        void document.exitFullscreen();
+      } else {
+        document.querySelector<HTMLElement>("[data-studio-fullscreen-target]")?.requestFullscreen();
+      }
+      return;
+    }
+
     // Delete / Backspace — remove selected element (timeline clip or preview selection)
     if (
       (event.key === "Delete" || event.key === "Backspace") &&