fix: allow closing last session tab and restore DevTools shortcut

Closing the sole remaining non-welcome tab now resets to welcome; enable Tauri devtools, menu/shortcut toggle, and copy release artifacts from CARGO_TARGET_DIR.

Co-authored-by: Cursor <cursoragent@cursor.com>

test/setup.ts

@@ -127,6 +127,7 @@ export const mockDesktopApi: DesktopApiContract = {
   clipboardReadText: vi.fn().mockResolvedValue(''),
   clipboardWriteText: vi.fn().mockResolvedValue(undefined),
   toggleMaximize: vi.fn().mockResolvedValue(undefined),
+  toggleDevtools: vi.fn().mockResolvedValue(false),
 
   // Local terminal
   localListShells: vi.fn().mockResolvedValue({