Add auto-reconnect with exponential backoff

Sessions configured with auto-reconnect will automatically attempt to
re-establish the connection when unexpectedly disconnected. Uses
exponential backoff (1s, 2s, 4s, 8s... up to 30s) with a configurable
maximum number of attempts (default 5).

- New SessionReconnecting state with pulsing indicator in tab bar
- SSH and Telnet profiles gain autoReconnect and maxReconnectTries fields
- RAW protocol already had autoReconnect; now uses the shared mechanism
- CancelReconnect API to abort in-progress reconnection
- Frontend shows reconnect attempt notifications
- Profile editor exposes toggle and max attempts in Advanced/Reconnection

internal/api/api.go

@@ -276,6 +276,11 @@ func (s *Service) ReconnectSession(sessionID string) error {
 	return s.sessions.Reconnect(sessionID, s.promptSecret)
 }
 
+// CancelReconnect cancels an in-progress auto-reconnect for a session.
+func (s *Service) CancelReconnect(sessionID string) {
+	s.sessions.CancelReconnect(sessionID)
+}
+
 // WriteSession writes base64-encoded data to a session.
 func (s *Service) WriteSession(sessionID string, data string) error {
 	decoded, err := base64.StdEncoding.DecodeString(data)