feat: add timeline navigation return to start functionality

- Add Enter key shortcut for seeking to timeline start (00:00:00)
- Add return to start button in timeline toolbar with skip-back icon
- Both methods complement existing Home key shortcut
- Update keyboard shortcuts help to display Enter key properly
- Maintain consistent UI styling with existing playback controls

Resolves #415

apps/web/src/stores/keybindings-store.ts

@@ -17,6 +17,7 @@ export const defaultKeybindings: KeybindingConfig = {
   "shift+left": "jump-backward",
   "shift+right": "jump-forward",
   home: "goto-start",
+  enter: "goto-start",
   end: "goto-end",
   s: "split-element",
   n: "toggle-snapping",