feat(api,ui): browse mode — paginated grid view per integration

[strausmann]

Implements ADR 0013 — browse mode part.

Backend

• GET /api/lookup/snipeit?page=1&page_size=24&category=…&sort=name (paginated list)
• GET /api/lookup/grocy?page=1&page_size=24&location=…&sort=name
• GET /api/lookup/spoolman?page=1&page_size=24&material=…&sort=name
• All return {items: [...], page, page_size, total, next_cursor?} with stable ordering
• Filters per integration (Snipe-IT category, Grocy location, Spoolman material/vendor)

Frontend

• Browse tab UI with grid layout (mobile 2-col, desktop 4-col)
• Tile shows: image (or placeholder), title, ID
• Infinite scroll OR pagination controls
• Tap a tile → item detail view (separate issue: detail view + layout dropdown)
• Loading states, empty states, error states

Acceptance

• Sort is stable (no duplicate items between pages)
• Pagination works on phone + desktop
• Empty state shows helpful message