feat(desktop): 顶栏租户切换器（多租户）—— 切换活跃租户即换工作区+计费

用户属多个租户时，顶栏显示租户下拉（Building2 图标）：切换即调 POST /me/tenant
换活跃租户，随后刷新余额芯片（显示新计费租户的可花余额）。仅 >1 租户时出现。
api 加 myTenants/switchTenant。

live 验证(preview)：demoB(属公司A+公司B) 切换器显两租户；从公司A(共享,余额15.84)
切到公司B(个人,余额6.38)→ 后端 active 持久化、芯片随之更新为 6.4(橙,偏低)。tsc+48 测试全过。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

sundynix-desktop/frontend/src/App.tsx

@@ -15,7 +15,7 @@ import { Placeholder } from "./views/Placeholder";
 import { CommandPalette, type Command } from "./components/CommandPalette";
 import { UpdateBanner } from "./components/UpdateBanner";
 import { Login } from "./views/Login";
-import { submitTask, streamTokens, streamExec, taskStatus, listRuns, authMe, logout, tenantCurrent, type Identity, type AuthUser, type TenantCtx } from "./lib/api";
+import { submitTask, streamTokens, streamExec, taskStatus, listRuns, authMe, logout, tenantCurrent, myTenants, switchTenant, type Identity, type AuthUser, type TenantCtx, type MyTenant } from "./lib/api";
 import type { TaskDsl } from "./lib/dsl";
 import { emptyRun, type RunState } from "./lib/run";
 import { ToastProvider } from "./ui";
@@ -43,6 +43,7 @@ export default function App() {
   const [view, setView] = useState<ViewKey>("home");
   const [user, setUser] = useState<AuthUser | null>(null);
   const [tenant, setTenant] = useState<TenantCtx | null>(null);
+  const [tenants, setTenants] = useState<MyTenant[]>([]);
   const [authLoading, setAuthLoading] = useState(true);
   const identity = useMemo<Identity>(() => ({ userId: user?.id ?? "", sessionId: getSessionId() }), [user]);
   const [run, setRun] = useState<RunState>(emptyRun);
@@ -92,10 +93,25 @@ export default function App() {
     tenantCurrent()
       .then(setTenant)
       .catch(() => {});
+    myTenants()
+      .then((r) => setTenants(r.tenants))
+      .catch(() => {});
   }, []);
+  const onSwitchTenant = useCallback(
+    async (id: string) => {
+      try {
+        await switchTenant(id);
+        refreshTenant();
+      } catch {
+        /* ignore */
+      }
+    },
+    [refreshTenant],
+  );
   useEffect(() => {
     if (!user) {
       setTenant(null);
+      setTenants([]);
       return;
     }
     refreshTenant();
@@ -239,7 +255,7 @@ export default function App() {
         style={{ background: "radial-gradient(60% 100% at 50% 0%, rgba(124,92,246,0.10), transparent 70%)" }}
       />
       <UpdateBanner />
-      <TopBar user={user} tenant={tenant} onLogout={onLogout} onCommand={() => setCmdOpen(true)} onOpenUsage={() => setView("usage")} />
+      <TopBar user={user} tenant={tenant} tenants={tenants} onSwitchTenant={onSwitchTenant} onLogout={onLogout} onCommand={() => setCmdOpen(true)} onOpenUsage={() => setView("usage")} />
       <ApprovalBar run={run} />
       <div className="relative flex min-h-0 flex-1">
         <LeftNav active={view} onSelect={setView} />