Fix USDT swap insufficient balance bug

Root cause: Token symbol mismatch between TokenSwap component and WalletContext
- WalletContext stored balance with key 'USDT'
- TokenSwap used symbol 'wUSDT' for lookups
- This caused balances['wUSDT'] to return undefined
- Triggered false "insufficient balance" errors

Changes:
- Updated TokenSwap.tsx to use 'USDT' symbol consistently
- Fixed token symbol in AVAILABLE_TOKENS array
- Updated asset ID mapping in swap transaction logic
- Fixed swap history token display

Also includes:
- Added new DEX components (SwapInterface, PoolBrowser, etc.)
- Added TypeScript type definitions for DEX
- Added DEX utility functions
- Removed obsolete test scripts
- Updated WalletContext with USDT balance support

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/pages/ProfileSettings.tsx

@@ -46,7 +46,7 @@ export default function ProfileSettings() {
         .from('profiles')
         .select('*')
         .eq('id', user?.id)
-        .single();
+        .maybeSingle();
 
       if (error) {
         console.error('Error loading profile:', error);