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-tauri/src/lib.rs

@@ -610,6 +610,25 @@ async fn open_in_explorer(state: State<'_, AppState>, target: String) -> Result<
     Ok(())
 }
 
+/// Open a local file with its associated application (e.g. launch Unreal for a
+/// `.uproject`). Uses the shell `start` so there are no path-scope restrictions.
+#[tauri::command]
+async fn launch_file(path: String) -> Result<(), String> {
+    if !std::path::Path::new(&path).exists() {
+        return Err(format!("File not found: {path}"));
+    }
+    let mut c = Command::new("cmd");
+    #[cfg(windows)]
+    {
+        use std::os::windows::process::CommandExt;
+        c.creation_flags(0x0800_0000);
+    }
+    c.args(["/C", "start", "", &path])
+        .spawn()
+        .map_err(|e| format!("Could not launch: {e}"))?;
+    Ok(())
+}
+
 /// Look for an Unreal `.uproject` inside the working-folder scope (resolved to a
 /// local dir via `p4 where`). Checks the folder itself and one level of subdirs.
 /// Returns the local path of the first match, or "" if the scope isn't a UE project.
@@ -751,6 +770,7 @@ pub fn run() {
             p4_undo_change,
             open_in_explorer,
             find_uproject,
+            launch_file,
             p4_describe,
             read_file,
             read_depot,