feat: AI cost tracking, pre-launch balance check, low balance alerts

- store/ai_charge.go: local AI cost tracking per call (SQLite)
- wallet/usdc.go: shared USDC balance query (Base chain RPC)
- Pre-launch: estimate daily cost + runway days
- Low balance: warn <$1, error at $0 (every 10 cycles)
- API: GET /api/ai-costs for cost history
- Frontend: model cards show price per call
- Frontend: wallet create + QR deposit + balance display

wallet/usdc.go

@@ -0,0 +1,91 @@
+// Package wallet provides shared wallet utilities (USDC balance queries, etc.)
+package wallet
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"math/big"
+	"net/http"
+	"strings"
+	"time"
+)
+
+const (
+	BaseRPCURL       = "https://mainnet.base.org"
+	USDCContractBase = "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913"
+	USDCDecimals     = 6
+)
+
+// QueryUSDCBalance queries USDC balance on Base chain and returns as float64
+func QueryUSDCBalance(address string) (float64, error) {
+	balanceStr := QueryUSDCBalanceStr(address)
+	var balance float64
+	_, err := fmt.Sscanf(balanceStr, "%f", &balance)
+	if err != nil {
+		return 0, fmt.Errorf("failed to parse balance: %w", err)
+	}
+	return balance, nil
+}
+
+// QueryUSDCBalanceStr queries USDC balance on Base chain and returns as formatted string
+func QueryUSDCBalanceStr(address string) string {
+	// Build balanceOf(address) call data
+	// Function selector: 0x70a08231
+	addrNoPre := strings.TrimPrefix(strings.ToLower(address), "0x")
+	data := "0x70a08231" + fmt.Sprintf("%064s", addrNoPre)
+
+	payload := map[string]interface{}{
+		"jsonrpc": "2.0",
+		"method":  "eth_call",
+		"params": []interface{}{
+			map[string]string{
+				"to":   USDCContractBase,
+				"data": data,
+			},
+			"latest",
+		},
+		"id": 1,
+	}
+
+	body, err := json.Marshal(payload)
+	if err != nil {
+		return "0.00"
+	}
+
+	client := &http.Client{Timeout: 10 * time.Second}
+	resp, err := client.Post(BaseRPCURL, "application/json", bytes.NewReader(body))
+	if err != nil {
+		return "0.00"
+	}
+	defer resp.Body.Close()
+
+	respBody, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return "0.00"
+	}
+
+	var rpcResp struct {
+		Result string `json:"result"`
+	}
+	if err := json.Unmarshal(respBody, &rpcResp); err != nil {
+		return "0.00"
+	}
+
+	// Parse hex result
+	hexStr := strings.TrimPrefix(rpcResp.Result, "0x")
+	if hexStr == "" || hexStr == "0" {
+		return "0.00"
+	}
+
+	balance := new(big.Int)
+	balance.SetString(hexStr, 16)
+
+	// Convert to float with 6 decimals
+	divisor := new(big.Int).Exp(big.NewInt(10), big.NewInt(USDCDecimals), nil)
+	whole := new(big.Int).Div(balance, divisor)
+	remainder := new(big.Int).Mod(balance, divisor)
+
+	return fmt.Sprintf("%d.%06d", whole, remainder)
+}