linux: open tray app on activation

[ousamabenyounes]

Summary

• Open the Linux Rust app window from the tray icon activation callback.
• Reuse the existing Open Window menu behavior for left-click activation.
• Add a focused unit test for the tray activation message.

Issues

Fix #636

Test verification (RED -> GREEN)

RED before implementation:

running 1 test
test ui::tray::tests::tray_activation_opens_window ... FAILED
assertion failed: matches!(ui_rx.try_recv(), Ok(BluetoothUIMessage::OpenWindow))

GREEN with fix:

running 1 test
test ui::tray::tests::tray_activation_opens_window ... ok
test result: ok. 1 passed; 0 failed

Revert proof:

running 1 test
test ui::tray::tests::tray_activation_opens_window ... FAILED
assertion failed: matches!(ui_rx.try_recv(), Ok(BluetoothUIMessage::OpenWindow))

Final checks:

cargo test ui::tray::tests::tray_activation_opens_window -- --exact
test result: ok. 1 passed; 0 failed

cargo test --no-run
Finished test profile