fix(web): harden ProductOffersExplorer, ProductShareAuditButton, and ProductValueCalculator bounds #1055

Problem

1. \�pps/web/src/components/leaderboard/ProductOffersExplorer.tsx:
   • \isSafeExternalOfferUrl(url)\ only checks \parsed.protocol === 'http:' || parsed.protocol === 'https:', allowing URLs with control characters, embedded credentials (\username/\password), or oversized lengths (>2048 chars).
   • \handleCopyOffersMarkdown\ does not strip control characters from \productName, \platform, \merchant.name, or \priceSpreadAudit\ merchant fields, and \safeSlugFileToken\ does not strip control characters or cap length.
   • \dosafe_risk_score\ and
     eview_risk_score\ are rendered and exported to CSV without clamping to [0, 100].
2. \�pps/web/src/components/leaderboard/ProductShareAuditButton.tsx:
   • 
     ormalizeShareAuditFields\ does not cap \safeAuthenticReviews\ or \safeOfferCount\ upper bounds (<= 10_000_000), nor strip control characters / cap length (<= 80) on \safeModelKeyToken.
   • 
     ame, \�rand, \modelKey, and \keySpecLine\ are interpolated into Markdown and JSON exports without stripping control characters.
3. \�pps/web/src/components/leaderboard/ProductValueCalculator.tsx:
   • \safeCategoryFileToken\ and \safeValueCriterionScore5\ are computed inline without exported helpers or control-character stripping / length bounds (<= 80\ chars).

Acceptance Criteria

• Harden \isSafeExternalOfferUrl\ and export \sanitizeOffersMarkdownCell, \sanitizeOffersFileToken, and \sanitizeOfferRiskScore100\ in \ProductOffersExplorer.tsx.
• Harden
  ormalizeShareAuditFields\ and export \sanitizeShareAuditText\ in \ProductShareAuditButton.tsx.
• Export \sanitizeValueCalcFileToken\ and \sanitizeValueCriterionScore5\ in \ProductValueCalculator.tsx\ and apply them to TCO calculations and exports.
• Add unit test suite [126h/135]\ in \core-rate-architecture.test.ts.