Replace hand-rolled async fetching (fetchFontsByIds + isRestoring flag)
with BatchFontStore backed by TanStack Query. Three reactive effects
handle batch sync, CSS font loading, and default-font fallback.
isLoading now derives from batchStore.isLoading + fontsReady.
- queryKeys: add mutation-safety test for batch(), key hierarchy tests
(list/batch/detail keys rooted in their parent base keys), and
unique-key test for different detail IDs
- BaseQueryStore: add initial-state test (data undefined, isError false
before any fetch resolves)
- BatchFontStore: add FontResponseError type assertion on malformed
response, null error assertion on success, and setIds([]) disables
query and returns empty fonts without triggering a fetch
