Add budget-constrained restocking with submitted order tracking

[andercsantos]

Summary

Adds a Restocking page that recommends which below-reorder-point items to replenish within an available budget, and surfaces submitted restock orders on the Orders page.

Changes

Frontend

• New Restocking.vue view + useRestocking.js composable — budget slider ($5K–$250K), greedy allocation by priority, and an explicit "excluded, over budget" table so skipped items stay visible rather than being silently dropped
• Submitted restock orders section on Orders, fetched in parallel with the filtered order list (restock orders take no filter params, so they stay visible across filter changes)
• Route + nav tab wiring; getRestockOrders / createRestockOrder in api.js
• en/ja locale strings

Backend

• GET / POST /api/restock-orders. POST recomputes the basket total server-side, so the budget check can't be bypassed by a tampered request
• Order ETA uses the longest lead time in the basket, since the order is only complete once its slowest line arrives
• lead_time_days added to InventoryItem (defaults to 14 when absent), populated across inventory.json

Also fixed

• Demand.vue rendered ++50.0% on the increasing-trend card — getChangePercent already returns a signed string and the template added a second +. Separate commit.

Testing

• 56/56 backend tests pass, including 16 new in tests/backend/test_restocking.py covering budget enforcement, item validation, and lead-time math
• All 7 pages verified in Chromium by clicking through each nav tab: zero console errors, zero failed requests
• Demand fix confirmed in-browser: +50.0%, +20.0%, +18.8%, -30.0%

Note

The budget allocator is greedy by priority, so it can leave money unspent — a $50K budget allocates $49,472 while PSU-505 is excluded for being "over by $410". Reasonable for a demo, but worth flagging if backfilling cheaper items into the remainder is preferred.

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