Go-live gate: refuse unsafe live trading

Setting MODE=live alone no longer starts trading. check_live_ready requires an
active risk limit, MAX_POSITION_PCT < 1.0, LIVE_CONFIRMATION exactly
"I UNDERSTAND THE RISK", a minimum balance, and exchange credentials. Enforced
in Runtime.start_many (both start paths) and surfaced at GET /api/live-check;
the dashboard shows a banner and blocks the start buttons. Adds tests.

frontend/src/api.js

@@ -5,6 +5,7 @@ const http = axios.create({ baseURL: '/api', timeout: 10000 })
 export const getVersion = () => http.get('/version').then((r) => r.data)
 export const getStatus = () => http.get('/status').then((r) => r.data)
 export const getMultiStatus = () => http.get('/bot/status').then((r) => r.data)
+export const getLiveCheck = () => http.get('/live-check').then((r) => r.data)
 export const startMany = (payload) => http.post('/bot/start-many', payload).then((r) => r.data)
 export const getStrategies = () => http.get('/strategies').then((r) => r.data)
 export const getPositions = () => http.get('/positions').then((r) => r.data)