Add Radar DC Telegram functionality and configuration

- Introduced new `RadarConfig` structure in `config.go` to manage radar settings, including `statuses_url`, `http_timeout_ms`, and `ping_from`.
- Implemented validation for radar configuration in `config_test.go` to ensure correct URL schemes and timeout limits.
- Added new API routes for radar statuses and ping functionality in the gateway, enhancing the service's capabilities.
- Updated documentation in `GATEWAY_RUN.md` to include details about the new radar features and their usage.
- Enhanced the user interface to include navigation and display options for the Radar DC section in the sidebar and page titles.
- Added client-side API functions for fetching radar statuses and ping responses, improving integration with the frontend.

config.example.yaml

@@ -43,6 +43,12 @@ trusted_proxies: []
 #     - gt2
 #   cache_ttl_ms: 2000
 
+# Радар DC Telegram: прокси к radar.telemt.top и TCP-пинг DC с хоста шлюза (см. README).
+# radar:
+#   statuses_url: "https://radar.telemt.top/api/v1/endpoints/statuses"
+#   http_timeout_ms: 30000
+#   ping_from: "stats.example.com"
+
 # Геолокация IP в /api/agg/unique-ips. См. docs/GEOIP.md (City + опционально ASN; Country-only не нужен)
 # geoip:
 #   enabled: true