Fix contract resolver: use Yahoo continuous contract price matching

Replace volume-based contract selection with Yahoo Finance continuous
contract price matching ({PRODUCT}=F). The old approach compared volumes
across front/roll candidates, which failed when serial months had
deceptive volume (6EJ26 > 6EM26) or Yahoo was rate-limited (all 0s).

Now fetches the continuous contract price and matches it against
candidates within 0.1% tolerance. Falls back to roll1 if Yahoo fails.
Also adds User-Agent header to avoid 429 rate limiting.

Verified: GC=F price matches GCM26, 6E=F price matches 6EM26.

Also fixes stale 'Random' comment in auto-trade.ts and cleans up
frontVolume/rolledVolume references from settings page.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/auto-trade.ts

@@ -119,7 +119,7 @@ export async function runTrade(action: 'Buy' | 'Sell' | 'Auto', symbol: string)
         resolvedSymbol = enabled.length > 0 ? enabled[Math.floor(Math.random() * enabled.length)] : 'NQ';
         console.log(`[auto-trade] random symbol resolved to: ${resolvedSymbol}`);
     }
-    // Resolve Random action once per batch so all accounts trade the same direction
+    // Resolve Auto action once per batch so all accounts trade the same direction
     const resolvedAction: 'Buy' | 'Sell' = action === 'Auto'
         ? (Math.random() < 0.5 ? 'Buy' : 'Sell')
         : action;