Add collapsible sidebar nav and Restocking feature

[LoganDavidTalbot]

Summary

• Sidebar navigation: Replaced the top nav bar with a collapsible left sidebar. Collapse state is persisted to localStorage and the sidebar auto-collapses on mobile viewports (<768px).
• Restocking page (/restocking): New view with a budget input that fetches demand-forecast-based recommendations from the backend. Items are greedy-selected within budget, sorted by trend urgency (increasing → stable → decreasing). Users can submit selected items as a restocking order.
• Orders tab integration: Submitted restocking orders appear in a dedicated section at the bottom of the Orders view.
• Backend: Three new FastAPI endpoints — GET /api/restocking/recommendations?budget=N, POST /api/restocking/orders, GET /api/restocking/orders. Orders are held in an in-memory list for the session.
• Docs/screenshots: Added architecture HTML doc and per-view PNG screenshots.

Test plan

• Sidebar collapses/expands via the toggle button; state survives a page refresh
• Sidebar auto-collapses on a narrow viewport and re-expands on wide viewport
• /restocking page loads; entering a budget populates recommendations with correct item costs
• Submitting a restocking order creates an entry visible in the Orders tab
• Existing routes (Dashboard, Inventory, Orders, Finance, Demand, Reports) still navigate correctly
• Backend: GET /api/restocking/recommendations?budget=50000 returns sorted candidates with selected flag
• Backend: POST /api/restocking/orders returns the new order and GET /api/restocking/orders lists it

🤖 Generated with Claude Code