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>
