feat: server-side launch preflight with structured checks and runtime AI wallet health

- POST /api/launch/preflight and GET /api/traders/:id/preflight run every
  launch prerequisite (AI model credential, claw402 wallet key + Base USDC
  balance, strategy, exchange config, live account state, funding minimums)
  and return a structured checklist with stable codes; minimums (1 USDC AI
  fee / 12 USDC trading) now live server-side and are exposed in the response.
- POST /traders/:id/start enforces the preflight (400 with error_key
  trader.start.preflight_failed + full checklist; ?force=true to override).
  Funding gates use max(available, equity) so restarting a bot with deployed
  margin is not blocked; indeterminate probes (RPC outage) degrade to
  warnings instead of blocking.
- Run loop now surfaces runtime health via GetStatus: safe_mode(+reason) and
  ai_wallet_status/balance (ok|low|empty|unknown), including typed detection
  of claw402 ErrInsufficientFunds instead of burying it in logs; safe-mode
  state is mutex-guarded for API readers.
- Onboarding wallet endpoints switch to the cached, error-aware balance query
  and report balance_status so the UI can tell an RPC outage from an empty
  wallet.

api/handler_trader.go

@@ -850,6 +850,24 @@ func (s *Server) handleStartTrader(c *gin.Context) {
 		return
 	}
 
+	// Server-side launch gate: the trader cannot function without a funded AI
+	// wallet and a ready exchange account, so verify both before the run loop
+	// starts. `?force=true` skips the gate for deliberate manual overrides.
+	if c.Query("force") != "true" {
+		// strategyRequired=false: a trader that loaded into memory necessarily
+		// has a valid strategy (the manager refuses to load without one), so the
+		// preflight strategy check would be redundant here.
+		preflight := s.runLaunchPreflight(userID, fullCfg.AIModel, fullCfg.Exchange, fullCfg.Strategy, false)
+		if !preflight.Ready {
+			c.JSON(http.StatusBadRequest, gin.H{
+				"error":     formatTraderStartError(preflight.Summary(), "Complete the failing checks, then start the bot again"),
+				"error_key": "trader.start.preflight_failed",
+				"preflight": preflight,
+			})
+			return
+		}
+	}
+
 	// Start trader
 	go func() {
 		logger.Infof("▶️  Starting trader %s (%s)", traderID, trader.GetName())