Add Linear-style quick filters to the library

Commit notes

Introduce a reusable quick-filter API plus a chip row above the sample grid in the library. The first quick filter is "Favourited".

Backend: - New services/quick-filters.ts defining quick-filter shortcuts (id, label, library query params) with live per-user counts. Adding a filter is a single registry entry plus a count condition. - New GET /api/v1/library/quick-filters endpoint returning the definitions with counts, so the same shortcuts can be reused anywhere.

Frontend: - getQuickFilters() API client + createQuickFiltersQuery() hook + query key. - LibraryQuickFilters.svelte renders Linear-style toggle chips (icon, label, count) above the grid. - library-state tracks active quick filters generically and merges their query params into getFilters(); counts as active filters and clears with the rest. - Favourite toggle invalidates the quick-filter counts.

[private session redacted]