Add Restocking Planner feature

[sum4every1]

Summary

• New Restocking view recommends items to reorder based on demand forecasts and current stock, with a budget cap and order submission flow
• Submitted restocking orders surface in the Orders view in a dedicated section alongside customer orders
• Backend gains GET/POST /api/restocking-orders endpoints (in-memory store, cleared on server restart)
• Adds en/ja i18n strings for the new view, architecture documentation, and per-page screenshots

Test plan

• npm run dev in client/ and uv run python main.py in server/
• Visit /restocking — recommendations render, budget gauge updates as items are selected
• Submit a restocking order — success message shows order number and expected delivery
• Visit /orders — submitted restocking orders appear in the "Submitted Restocking Orders" section
• Switch language to Japanese — all new strings localize correctly
• Restart server — restocking orders list resets (expected, in-memory)

\U0001F916 Generated with Claude Code