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_force.go

@@ -21,7 +21,7 @@ import (
 //   - never exceeds MaxPositions,
 //   - never doubles a base symbol already held or already in the decision set.
 func (at *AutoTrader) ensureLongShortCoverage(decisions []kernel.Decision, ctx *kernel.Context, equity float64) []kernel.Decision {
-	if at == nil || ctx == nil || at.safeMode {
+	if at == nil || ctx == nil || at.isSafeMode() {
 		return decisions
 	}
 	if at.config.StrategyConfig == nil || at.config.StrategyConfig.CoinSource.SourceType != "vergex_signal" {