dashboard: show GATEWAY_HEALTH_URL instead of PID for remote gateways

When the dashboard connects to a remote gateway via GATEWAY_HEALTH_URL,
display the URL instead of the remote PID (which is meaningless locally).
Falls back to PID display for local gateways as before.

- Backend: expose gateway_health_url in /api/status response
- Frontend: prefer gateway_health_url over PID in gatewayValue()
- Add truncate + title tooltip for long URLs that overflow the card
- Add min-w-0/overflow-hidden on status cards for proper truncation
- Tests: verify gateway_health_url in remote and no-URL scenarios

web/src/lib/api.ts

@@ -213,6 +213,7 @@ export interface StatusResponse {
   config_version: number;
   env_path: string;
   gateway_exit_reason: string | null;
+  gateway_health_url: string | null;
   gateway_pid: number | null;
   gateway_platforms: Record<string, PlatformStatus>;
   gateway_running: boolean;