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/mod.rs

@@ -1,6 +1,7 @@
 #![allow(non_snake_case)]
 
 mod auth;
+mod balance;
 mod coding_plan;
 mod config;
 mod copilot;
@@ -31,6 +32,7 @@ mod webdav_sync;
 mod workspace;
 
 pub use auth::*;
+pub use balance::*;
 pub use coding_plan::*;
 pub use config::*;
 pub use copilot::*;