Desktop: reopen hidden macOS window from Dock

[wasimysaid]

Summary

• handle macOS Dock activation through Tauri RunEvent::Reopen
• restore and focus the hidden main window through the shared Dock, tray, and single-instance path
• add regression coverage for the Dock reopen lifecycle

Testing

• uvx --from pytest pytest tests/studio/test_desktop_reliability_frontend_contract.py -q (13 passed)
• npm run build --prefix studio/frontend
• cargo check --manifest-path studio/src-tauri/Cargo.toml --locked
• rustfmt --edition 2021 --check < studio/src-tauri/src/main.rs

Not run

• packaged macOS smoke test, since the current host is Linux