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.

trader/auto_trader.go

@@ -177,8 +177,12 @@ type AutoTrader struct {
 	gridState             *GridState         // Grid trading state (only used when StrategyType == "grid_trading")
 	claw402WalletAddr     string             // Claw402 wallet address (derived from private key at start)
 	consecutiveAIFailures int                // Consecutive AI call failures
+	runtimeHealthMu       sync.RWMutex       // Guards safe mode + AI wallet health (loop writes, API reads)
 	safeMode              bool               // Safe mode: no new positions, protect existing ones
 	safeModeReason        string             // Why safe mode was activated
+	aiWalletStatus        string             // "ok"|"low"|"empty"|"unknown" — see runtime_health.go
+	aiWalletBalanceUSDC   float64            // Last observed Base USDC balance of the claw402 wallet
+	aiWalletCheckedAt     time.Time          // When the balance was last observed
 }
 
 // NewAutoTrader creates an automatic trader
@@ -710,7 +714,9 @@ func (at *AutoTrader) runPreLaunchChecks() {
 			balance, err := wallet.QueryUSDCBalance(addr)
 			if err != nil {
 				logger.Warnf("⚠️ [%s] Could not query USDC balance: %v", at.name, err)
+				at.markAIWalletHealthUnknown()
 			} else {
+				at.setAIWalletHealth(balance)
 				// Estimate runway
 				scanMinutes := int(at.config.ScanInterval.Minutes())
 				modelName := at.config.CustomModelName