Use graceful Tauri shutdown instead of hard process exit

Replace std::process::exit(0) with app_handle.exit(0) in the tray
quit handler so Tauri runs its shutdown sequence, dropping resources
and firing RunEvent::Exit cleanly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -172,7 +172,7 @@ pub fn run() {
                         }
                     }
                     "quit" => {
-                        std::process::exit(0);
+                        app_handle.exit(0);
                     }
                     _ => {}
                 })