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/wailsjs/go/api/Service.js

@@ -126,6 +126,10 @@ export function ReconnectSession(arg1) {
   return window['go']['api']['Service']['ReconnectSession'](arg1);
 }
 
+export function RegisterSecretResponseListener() {
+  return window['go']['api']['Service']['RegisterSecretResponseListener']();
+}
+
 export function RenameSession(arg1, arg2) {
   return window['go']['api']['Service']['RenameSession'](arg1, arg2);
 }
@@ -170,6 +174,10 @@ export function SetProfileSecret(arg1, arg2, arg3) {
   return window['go']['api']['Service']['SetProfileSecret'](arg1, arg2, arg3);
 }
 
+export function TestConnection(arg1, arg2) {
+  return window['go']['api']['Service']['TestConnection'](arg1, arg2);
+}
+
 export function UnlockSecretStore(arg1) {
   return window['go']['api']['Service']['UnlockSecretStore'](arg1);
 }