feat(pricing): construct detailed feature comparison table matrix

[noevidence1017]

Closes #305

Implements the four-column pricing feature comparison table.

• Feature column plus Starter / Business / Enterprise columns
• Rows include Blockchain Confirmation, Escrow Multi-Sig, Multi-Currency Settlement, Dispute Resolution, API Access, Dedicated Account Manager, and Uptime SLA
• Subtle row dividers and consistent cell spacing
• Table scrolls horizontally on narrow viewports via an overflow-x-auto wrapper
• Follows the Component -> Hook -> Service architecture: ComparisonTable.tsx -> usePricingComparison -> pricingService
• Data is fetched from /api/pricing/comparison (dev-only fixture route) rather than hardcoded
• Loading skeleton and error/retry state included
• 4 unit tests covering loading, error, rendered rows/columns, and the service response shape

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