fix(auth): 修复重启后无法对话 — restoreSession 优先验证 SaaS token

根因: 心跳降级将 'tauri' 持久化到 localStorage，重启后盲信该值。
修复: token refresh 成功时强制恢复 'saas' 模式；connectionMode 携带时间戳。

desktop/src/lib/saas-session.ts

@@ -157,16 +157,41 @@ export async function clearSaaSSession(): Promise<void> {
 }
 
 /**
- * Persist the connection mode to localStorage.
+ * Persist the connection mode to localStorage with timestamp.
  */
 export function saveConnectionMode(mode: string): void {
-  localStorage.setItem(SAASMODE_KEY, mode);
+  const data = JSON.stringify({ mode, timestamp: Date.now() });
+  localStorage.setItem(SAASMODE_KEY, data);
 }
 
 /**
  * Load the connection mode from localStorage.
+ * Handles both new JSON format and legacy plain string format.
  * Returns null if not set.
  */
 export function loadConnectionMode(): string | null {
-  return localStorage.getItem(SAASMODE_KEY);
+  const raw = localStorage.getItem(SAASMODE_KEY);
+  if (!raw) return null;
+  try {
+    const parsed = JSON.parse(raw);
+    if (typeof parsed === 'string') return parsed; // legacy format
+    return parsed.mode ?? null;
+  } catch {
+    return raw; // legacy format (plain string)
+  }
+}
+
+/**
+ * Load the timestamp of the persisted connection mode.
+ * Returns null if not set or format is legacy.
+ */
+export function loadConnectionModeTimestamp(): number | null {
+  const raw = localStorage.getItem(SAASMODE_KEY);
+  if (!raw) return null;
+  try {
+    const parsed = JSON.parse(raw);
+    return parsed.timestamp ?? null;
+  } catch {
+    return null;
+  }
 }

desktop/src/store/saasStore.ts

@@ -818,8 +818,11 @@ export const useSaaSStore = create<SaaSStore>((set, get) => {
         account,
         saasUrl: restored.saasUrl,
         authToken: newToken,
-        connectionMode: loadConnectionMode() === 'saas' ? 'saas' : 'tauri',
+        // If token refresh succeeded, always restore to 'saas' mode
+        // regardless of what was persisted (heartbeat may have degraded to 'tauri')
+        connectionMode: 'saas',
       });
+      saveConnectionMode('saas');
       get().fetchAvailableModels().catch(() => {});
       get().fetchAvailableTemplates().catch(() => {});
       get().fetchAssignedTemplate().catch(() => {});