Site-wide performance optimization and checkout correctness

[GautamBytes]

This PR delivers a single-pass, performance-only optimization across app shell, data loading, route chunking, parsing, and caching, plus final checkout correctness fixes.

Key outcomes:

• Reduced shared first-load JS from 394 kB baseline to 206 kB
• Reduced _app chunk from 835 kB raw baseline to 77.5 kB
• Route targets improved: /orders 197 kB, /listing 286 kB, /cart 266 kB
• Route-scoped bootstrap and lazy-loading of signer/nostr-heavy paths
• Startup/data fetch cleanup (less duplicate work, better staged loading)
• Route-level code splitting for heavy cart/listing/orders flows
• Parser/slug hot-path optimizations with bounded cache to avoid unbounded memory growth
• Polling reductions via event-driven storage updates
• PWA/service-worker tuning and middleware import hardening
• API/DB fetch scoping to reduce overfetching

[vercel]

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