Implement connection flow, test connection, and secret prompts

- Change ConnectSession/ConnectQuick to return SessionInfo (not string)
- Change UpdateProfile to return (Profile, error) for frontend use
- Make session connection asynchronous to avoid Wails event loop deadlock
- Implement event-based secret prompt (backend ↔ frontend via requestID)
- Add TestConnection API for TCP reachability checks with latency
- Add 'Save & Connect' button in profile editor
- Wire _testConnection to real backend TCP check
- Add btn-accent CSS for the green action button

frontend/src/js/app.js

@@ -185,12 +185,12 @@ class App {
                     payload.message || 'Enter your password or passphrase.',
                 );
                 window.runtime.EventsEmit('session:secret:response', {
-                    sessionID: payload.sessionID,
+                    requestID: payload.requestID,
                     value,
                 });
             } catch (_) {
                 window.runtime.EventsEmit('session:secret:response', {
-                    sessionID: payload.sessionID,
+                    requestID: payload.requestID,
                     value: '',
                     cancelled: true,
                 });