refactor(arch): Phase 2 — deduplicate isAdmin, convertToUsd, getBaseWalletType

- #54: Extract isAdmin() to src/middleware/auth.js, remove duplicates from 7 admin handlers
- #55: Add WalletUtils.convertToUsd(), replace 8 switch-case blocks across 4 files
- #56: Unify getBaseWalletType() — keep only WalletUtils version (most complete),
  remove duplicates from Wallet.js and userWalletsHandler.js

New file: src/middleware/auth.js
Net: -215 lines, +80 lines

Closes: #54, #55, #56

src/models/Wallet.js

@@ -4,11 +4,6 @@ import db from "../config/database.js";
 import WalletUtils from "../utils/walletUtils.js";
 
 export default class Wallet {
-    static getBaseWalletType(walletType) {
-        if (walletType.includes('ERC-20')) return 'ETH';
-        return walletType;
-    }
-
     static async getArchivedWallets(userId) {
         const archivedWallets = await db.allAsync(`
             SELECT * FROM crypto_wallets WHERE user_id = ? AND wallet_type LIKE '%_%'
@@ -60,7 +55,7 @@ export default class Wallet {
         let totalUsdBalance = 0;
 
         for (const [type, balance] of Object.entries(balances)) {
-            const baseType = this.getBaseWalletType(type);
+            const baseType = WalletUtils.getBaseWalletType(type);
             const wallet = activeWallets.wallets.find(w =>
                 w.wallet_type === baseType ||
                 (type.includes('ERC-20') && w.wallet_type === 'ETH')
@@ -92,7 +87,7 @@ export default class Wallet {
         let totalUsdBalance = 0;
 
         for (const [type, balance] of Object.entries(balances)) {
-            const baseType = this.getBaseWalletType(type);
+            const baseType = WalletUtils.getBaseWalletType(type);
             const wallet = archiveWallets.wallets.find(w =>
                 w.wallet_type === baseType ||
                 (type.includes('ERC-20') && w.wallet_type.startsWith('ETH'))