Add restocking feature and architecture docs

[Mcreve]

Summary

• Restocking tab: Budget slider ($1K-$200K) with demand-gap-based item recommendations, selectable quantities, and order submission
• Backend APIs: GET/POST /api/orders/restock for order management, GET /api/restock/recommendations for budget-aware suggestions
• Orders tab: Submitted restock orders section with lead time tracking
• i18n: Full English and Japanese support for all new UI elements
• Architecture page: HTML-based system architecture documentation

Test plan

• Verify Restocking tab loads with budget slider and 8 recommended items
• Select items, adjust quantities, and submit a restock order
• Navigate to Orders tab and confirm submitted order appears with correct details
• Switch to Japanese and verify all new translations render correctly
• Open architecture.html in browser and verify documentation renders

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