refactor(ui): move generic utilities to ui-common and add useFetchData composable

refactor(ui): move generic utilities to ui-common and add useFetchData composable

- Move convertToBoolean(), convertToInt() to ui-common/src/utils/converters.ts
- Move getWebSocketStateName() to ui-common/src/utils/websocket.ts using
  portable WebSocketReadyState enum instead of browser WebSocket constants
- Add useFetchData() composable to deduplicate fetch-with-loading-guard
  pattern in ChargingStationsView (3 instances collapsed)
- All consumers import directly from ui-common, no re-exports