feat(catalog): two-level category tree (subcategory UI) #1340

Parent: #830 (S5). Verified against main @ 8e39053.

Problem

Stores with large menus want subcategories, for example Drinks > Hot / Cold (+44, 21 Sep).

Current behavior (verified)

• The backend already supports it. categories.parent_id has been in the schema since the start (main/db.ts ~L7883). main/routes/categories.ts ~L111-140 validates the parent and blocks cycles on create and update. GET /categories?root and ?parent_id= exist and nest one level. Tests: tests/issue-247-catalog-invariants.test.ts.
• There is no UI. parent_id appears only in frontend/src/lib/types.ts. The category form (frontend/src/app/(dashboard)/products/page.tsx ~L695-701) never sends it, and POS loads a flat list (pos/page.tsx ~L1065).

Decisions (made)

• Max depth 2 (category > subcategory). The API should reject a parent that itself has a parent, which matches the one-level nesting GET already returns.
• POS: tapping a parent shows its subcategory chips and the parent's own products. "All" still shows everything.
• Sort order (feat(pos): configurable category and item sort order #1311) applies among siblings.

Scope

• Products page category form: an optional "Parent category" select limited to root categories, and an indented tree in the categories list.
• POS category bar and components/pos/ProductGrid.tsx: subcategory navigation.
• Menu CSV import/export (main/routes/menu-csv.ts): a parent column by name, resolved on import.
• Menu printing (main/printers/document-menu.ts): print subcategories under their parent.
• Backend: enforce depth 2 in categories.ts. Deleting or deactivating a parent with children must be refused or must re-parent the children (choose refuse, with a clear message).

Acceptance criteria

• Create a subcategory from the UI, and see it under its parent in management, POS and the printed menu.
• Setting a grandchild parent is rejected (400).
• A CSV round-trip preserves the hierarchy.
• Stores with no subcategories look exactly as before.

Verification

• Extend tests/issue-247-catalog-invariants.test.ts and the menu CSV tests, plus an e2e for create and POS navigation.
• npm run lint && npm test