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/shell/TopBar.tsx

@@ -1,6 +1,6 @@
 import type { CSSProperties } from "react";
-import { User, ChevronDown, Search as SearchIcon, LogOut, Sun, Moon, Coins } from "lucide-react";
-import type { AuthUser, TenantCtx } from "../lib/api";
+import { User, ChevronDown, Search as SearchIcon, LogOut, Sun, Moon, Coins, Building2 } from "lucide-react";
+import type { AuthUser, TenantCtx, MyTenant } from "../lib/api";
 import { useHealth } from "../lib/health";
 import { isMacDesktop } from "../lib/desktop";
 import { useTheme } from "../lib/theme";
@@ -42,8 +42,31 @@ function CreditChip({ tenant, onClick }: { tenant: TenantCtx; onClick?: () => vo
   );
 }
 
+// 租户切换器：仅当用户属多个租户时显示；切换即改活跃租户（工作区 + 计费上下文）。
+function TenantSwitcher({ tenants, activeId, onSwitch }: { tenants: MyTenant[]; activeId?: string; onSwitch?: (id: string) => void }) {
+  if (tenants.length < 2) return null;
+  return (
+    <div className="relative" style={NODRAG}>
+      <Building2 className="pointer-events-none absolute left-2 top-1/2 h-3.5 w-3.5 -translate-y-1/2 text-slate-500" />
+      <select
+        value={activeId ?? ""}
+        onChange={(e) => onSwitch?.(e.target.value)}
+        title="切换当前租户（工作区 + 计费）"
+        className="appearance-none rounded-md border border-line bg-ink-800 py-1 pl-7 pr-7 text-xs text-slate-300 focus:border-brand focus:outline-none"
+      >
+        {tenants.map((t) => (
+          <option key={t.id} value={t.id}>
+            {t.name}
+          </option>
+        ))}
+      </select>
+      <ChevronDown className="pointer-events-none absolute right-2 top-1/2 h-3.5 w-3.5 -translate-y-1/2 text-slate-500" />
+    </div>
+  );
+}
+
 // 顶栏：品牌 · 垂直切换 · 健康灯 · 积分余额 · 登录用户 + 登出（深色 + 毛玻璃）。
-export function TopBar({ user, tenant, onLogout, onCommand, onOpenUsage }: { user: AuthUser; tenant?: TenantCtx | null; onLogout: () => void; onCommand?: () => void; onOpenUsage?: () => void }) {
+export function TopBar({ user, tenant, tenants = [], onSwitchTenant, onLogout, onCommand, onOpenUsage }: { user: AuthUser; tenant?: TenantCtx | null; tenants?: MyTenant[]; onSwitchTenant?: (id: string) => void; onLogout: () => void; onCommand?: () => void; onOpenUsage?: () => void }) {
   const h = useHealth();
   const { theme, toggle } = useTheme();
   return (
@@ -86,6 +109,7 @@ export function TopBar({ user, tenant, onLogout, onCommand, onOpenUsage }: { use
         <Light on={h.neo4j} label="Neo4j" />
       </div>
       <div className="ml-auto flex items-center gap-2" style={NODRAG}>
+        <TenantSwitcher tenants={tenants} activeId={tenant?.tenant?.id} onSwitch={onSwitchTenant} />
         {tenant?.tenant && <CreditChip tenant={tenant} onClick={onOpenUsage} />}
         <button
           onClick={toggle}