Add Restocking feature and redesign navigation as a left sidebar

[mattj83]

Summary

• New Restocking tab: budget-driven restock recommendations sourced from demand forecasts, with a running-total budget slider and order placement flow
• Replaced the top nav bar with a collapsible left sidebar (AppSidebar.vue + useSidebar composable) for a more SaaS-style layout
• Backend support for restocking orders (server/data/restocking_orders.json, mock_data.py, main.py) and matching pytest coverage (tests/backend/test_restocking.py)
• Minor: pinned the local Playwright MCP config to chromium --isolated in .mcp.json so browser-based testing doesn't collide with other sessions

Test plan

• Backend and frontend dev servers start cleanly
• Manually walked through all nav tabs (Overview, Inventory, Orders, Restocking, Finance, Demand Forecast, Reports) via Playwright — all load without errors
• Restocking budget slider, recommendation table, and running total render correctly
• pytest tests/backend/test_restocking.py (run in CI)

Known issues found during manual testing (not introduced by this PR, worth follow-up)

• Every page logs a Failed to load tasks: 404 — some component polls a non-existent /api/tasks endpoint
• Overview dashboard: Failed to resolve component: PurchaseOrderModal Vue warning
• Overview dashboard: "Inventory Value by Category" bar labels clip/overflow for smaller values
• Demand Forecast: summary cards show doubled ++ sign on positive percentages
• Reports.vue has leftover console.log debug statements

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