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-tauri/src/commands/balance.rs

@@ -0,0 +1,6 @@
+use crate::provider::UsageResult;
+
+#[tauri::command]
+pub async fn get_balance(base_url: String, api_key: String) -> Result<UsageResult, String> {
+    crate::services::balance::get_balance(&base_url, &api_key).await
+}