Fix UE launch permission, move it to context menu, refresh History after Submit

- Launch now goes through a backend launch_file command (shell `start`), so
  it works for any path — fixes "Not allowed to open path" from the opener
  plugin's scoped open-path.
- Removed the toolbar Launch-Unreal button; it's now a right-click item on the
  Working folder ("Launch Unreal Engine"), shown only when a .uproject exists.
- Reload History after a successful Submit so the new changelist shows without
  a manual F5.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/p4.ts

@@ -73,6 +73,7 @@ export const p4 = {
   undoChange: (change: string) => invoke<string>("p4_undo_change", { change }),
   openInExplorer: (target: string) => invoke<void>("open_in_explorer", { target }),
   findUproject: (scope: string) => invoke<string>("find_uproject", { scope }),
+  launchFile: (path: string) => invoke<void>("launch_file", { path }),
   readDepot: async (depot: string): Promise<ArrayBuffer> => {
     const r: unknown = await invoke("read_depot", { depot });
     if (r instanceof ArrayBuffer) return r;