feat(proxy): implement per-app takeover mode

Replace global live takeover with granular per-app control:
- Add start_proxy_server command (start without takeover)
- Add get_proxy_takeover_status to query each app's state
- Add set_proxy_takeover_for_app for individual app control
- Use live backup existence as SSOT for takeover state
- Refactor sync_live_to_provider to eliminate code duplication
- Update ProxyToggle to show status per active app

src-tauri/src/database/dao/proxy.rs

@@ -85,15 +85,8 @@ impl Database {
 
     /// 检查是否处于 Live 接管模式
     pub async fn is_live_takeover_active(&self) -> Result<bool, AppError> {
-        let conn = lock_conn!(self.conn);
-        let active: i32 = conn
-            .query_row(
-                "SELECT COALESCE(live_takeover_active, 0) FROM proxy_config WHERE id = 1",
-                [],
-                |row| row.get(0),
-            )
-            .unwrap_or(0);
-        Ok(active != 0)
+        // v3.7.0+：以 proxy_live_backup 是否存在作为“接管状态”的真实来源（更贴近 per-app 接管）
+        self.has_any_live_backup().await
     }
 
     // ==================== Provider Health ====================