refactor(proxy): remove is_proxy_target in favor of failover_queue

- Remove `is_proxy_target` field from Provider struct (Rust & TypeScript)
- Remove related DAO methods: get_proxy_target_provider, set_proxy_target
- Remove deprecated Tauri commands: get_proxy_targets, set_proxy_target
- Add `is_available()` method to CircuitBreaker for availability checks
  without consuming HalfOpen probe permits (used in select_providers)
- Keep `allow_request()` for actual request gating with permit tracking
- Update stream_check to use failover_queue instead of is_proxy_target
- Clean up commented-out reset circuit breaker button in ProviderActions
- Remove unused useProxyTargets and useSetProxyTarget hooks

src-tauri/src/database/schema.rs

@@ -31,19 +31,12 @@ impl Database {
                 icon_color TEXT,
                 meta TEXT NOT NULL DEFAULT '{}',
                 is_current BOOLEAN NOT NULL DEFAULT 0,
-                is_proxy_target BOOLEAN NOT NULL DEFAULT 0,
                 PRIMARY KEY (id, app_type)
             )",
             [],
         )
         .map_err(|e| AppError::Database(e.to_string()))?;
 
-        // 尝试添加 is_proxy_target 列（如果表已存在但缺少该列）
-        let _ = conn.execute(
-            "ALTER TABLE providers ADD COLUMN is_proxy_target BOOLEAN NOT NULL DEFAULT 0",
-            [],
-        );
-
         // 2. Provider Endpoints 表
         conn.execute(
             "CREATE TABLE IF NOT EXISTS provider_endpoints (