feat: Implement percentage-based position sizing

- Add usePercentageSize flag to SymbolSettings and TradingConfig
- Add calculateActualPositionSize() and getActualPositionSizeForSymbol() helpers
- Update execute and test endpoints to calculate position size from free collateral
- Add SOLANA_USE_PERCENTAGE_SIZE, ETHEREUM_USE_PERCENTAGE_SIZE, USE_PERCENTAGE_SIZE env vars
- Configure SOL to use 100% of portfolio (auto-adjusts to available balance)
- Fix TypeScript errors: replace fillNotionalUSD with actualSizeUSD
- Remove signalQualityVersion and fullyClosed references (not in interfaces)
- Add comprehensive documentation in PERCENTAGE_SIZING_FEATURE.md

Benefits:
- Prevents insufficient collateral errors by using available balance
- Auto-scales positions as account grows/shrinks
- Maintains risk proportional to capital
- Flexible per-symbol configuration (SOL percentage, ETH fixed)

app/api/settings/route.ts

@@ -71,14 +71,17 @@ export async function GET() {
       // Global fallback
       MAX_POSITION_SIZE_USD: parseFloat(env.MAX_POSITION_SIZE_USD || '50'),
       LEVERAGE: parseFloat(env.LEVERAGE || '5'),
+      USE_PERCENTAGE_SIZE: env.USE_PERCENTAGE_SIZE === 'true',
       
       // Per-symbol settings
       SOLANA_ENABLED: env.SOLANA_ENABLED !== 'false',
       SOLANA_POSITION_SIZE: parseFloat(env.SOLANA_POSITION_SIZE || '210'),
       SOLANA_LEVERAGE: parseFloat(env.SOLANA_LEVERAGE || '10'),
+      SOLANA_USE_PERCENTAGE_SIZE: env.SOLANA_USE_PERCENTAGE_SIZE === 'true',
       ETHEREUM_ENABLED: env.ETHEREUM_ENABLED !== 'false',
       ETHEREUM_POSITION_SIZE: parseFloat(env.ETHEREUM_POSITION_SIZE || '4'),
       ETHEREUM_LEVERAGE: parseFloat(env.ETHEREUM_LEVERAGE || '1'),
+      ETHEREUM_USE_PERCENTAGE_SIZE: env.ETHEREUM_USE_PERCENTAGE_SIZE === 'true',
       
       // Risk management
       STOP_LOSS_PERCENT: parseFloat(env.STOP_LOSS_PERCENT || '-1.5'),