feat: add official balance query for DeepSeek, StepFun, SiliconFlow, OpenRouter, Novita AI

Add a new "Official" (官方) template type in the usage query panel that
queries account balance via each provider's native API endpoint.
Follows the same zero-script pattern as Token Plan — Rust handles the
HTTP call, frontend auto-detects the provider from base URL.

Supported providers and endpoints:
- DeepSeek: GET /user/balance
- StepFun: GET /v1/accounts
- SiliconFlow: GET /v1/user/info (cn + com)
- OpenRouter: GET /api/v1/credits
- Novita AI: GET /v3/user/balance

src/lib/api/subscription.ts

@@ -9,4 +9,9 @@ export const subscriptionApi = {
     apiKey: string,
   ): Promise<SubscriptionQuota> =>
     invoke("get_coding_plan_quota", { baseUrl, apiKey }),
+  getBalance: (
+    baseUrl: string,
+    apiKey: string,
+  ): Promise<import("@/types").UsageResult> =>
+    invoke("get_balance", { baseUrl, apiKey }),
 };