fix: division-by-zero guard, rate-limit wallet endpoints, safer JSON parse, Lighter HTTP checks

- Guard GridCount-1 division by zero in grid spacing calculation (both grid.go and grid_levels.go)
- Rate-limit /wallet/validate and /wallet/generate endpoints (prevent DoS on key generation)
- Wrap JSON.parse(localStorage) in try-catch in AuthContext (corrupted data → graceful re-login)
- Wrap handleJSONResponse JSON.parse in try-catch with descriptive error
- Add HTTP status code checks for Lighter GetActiveOrders and sendTx
- Replace deprecated strings.Title with cases.Title (golang.org/x/text)
- Reuse HTTP client in checkClaw402Health (was creating new client per call)

api/handler_wallet.go

@@ -115,9 +115,10 @@ func (s *Server) handleWalletGenerate(c *gin.Context) {
 	})
 }
 
+var walletHTTPClient = &http.Client{Timeout: 5 * time.Second}
+
 func checkClaw402Health() string {
-	client := &http.Client{Timeout: 5 * time.Second}
-	resp, err := client.Get("https://claw402.ai/health")
+	resp, err := walletHTTPClient.Get("https://claw402.ai/health")
 	if err != nil {
 		return "unreachable"
 	}