Add Restocking feature with demand-based order planning

[unmeshk75]

Summary

• Adds a new Restocking tab with a demand forecast planner: users can review forecasted items, adjust quantities, and submit restocking orders
• New backend endpoints (POST /api/restocking-orders, GET /api/restocking-orders) backed by an in-memory list in mock_data.py
• Submitted restocking orders appear pinned at the top of the Orders view with expandable item details, lead time range, and total value
• Extended DemandForecast Pydantic model with unit_cost; updated demand_forecasts.json to include unit costs

Test plan

• Navigate to /restocking — planner table loads with demand forecast items
• Select items, adjust quantities, and click Submit Order — success toast appears
• Go to /orders — submitted restocking orders appear in the amber-bordered section at the top
• Verify GET /api/restocking-orders returns the submitted order via /docs
• Reload the page — in-memory orders persist for the server session

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[unmeshk75]

Created this PR through the GitHub app in Claude at the Basecamp!

[unmeshk75]

Basecamp testing