feat: phone login via QR when the saved Mudfish login expires

When autologin is rejected the panel offers '휴대폰으로 로그인 (QR)'. It opens
a 5-minute LAN relay to the launcher UI guarded by a one-time token (first
phone to open the QR URL is the only client allowed) and shows the URL as a
QR code. The relay closes as soon as a new session appears, on cancel, on
expiry, and on unload.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/backend.ts

@@ -17,6 +17,8 @@ export interface VpnState {
   country: string | null;
   connected_since: number | null;
   tunnel: boolean;
+  need_login: boolean;
+  login: { url: string; expires_at: number } | null;
 }
 
 export const getState = callable<[], VpnState>("get_state");
@@ -24,3 +26,5 @@ export const connect = callable<[], VpnState>("connect");
 export const disconnect = callable<[], VpnState>("disconnect");
 export const setNode = callable<[choice: string], VpnState>("set_node");
 export const listNodes = callable<[], NodeInfo[]>("list_nodes");
+export const startLogin = callable<[], VpnState>("start_login");
+export const stopLogin = callable<[], VpnState>("stop_login");