Add budget-aware restocking

[Manish567Kumar]

Adds a Restocking tab that recommends what to reorder and fits it to an available budget, plus a Submitted Orders table on the Orders tab.

Backend (server/main.py):

• GET /api/restocking/candidates - prices every item needing reorder and ranks urgent-then-cheapest so a small budget clears critical shortfalls first. Returns full costs so the client can apply the budget reactively.
• POST /api/restock-orders - validates the total against the budget and rejects an over-budget submission.
• GET /api/restock-orders - submitted orders, newest first.
• Restock orders are held separately from orders so dashboard revenue and fulfilment aggregates are not inflated by demo submissions.

Frontend:

• Restocking.vue - budget slider drives a greedy fill in a computed, so dragging never re-hits the API. Funded and not-funded items shown separately.
• Orders.vue - Submitted Orders table with a guarded date formatter; a failing restock feed cannot blank out the customer orders table.