frontend: add test coverage for hooks and API layer

[nitrobass24]

Depends on

#1963. Do not start work here until enough seams have been extracted for the hooks listed below to be testable with renderHook without massive mock setups. Without that prerequisite, the tests this issue produces will either be trivial (covering already-pure surface) or brittle (RTL with many mocked providers per test). See discussion on #1927.

Scope clarification

The decomposition work extracts new hooks from TorrentTableOptimized.tsx (useTorrentSelection, useTorrentSelectionDerivations, useTorrentTableFilterExpr, useCompactViewSort, useCrossSeedOrchestration, useTorrentTableHotkeys, useBulkActionWrappers, useTorrentTableNotifications, useTorrentTableColumns, useTorrentTableVirtualization, useFilterLifecycle, useColumnDragAndDrop). Those new hooks are tested in their own decomposition PRs and are NOT covered by this issue.

This issue's scope is the pre-existing big hooks (useTorrentActions, useTorrentsList, useCrossSeedSearch, useTorrentExporter, useSpeedUnits, useLocalCrossSeedMatches, usePersistedFilters) plus lib/api.ts contract tests via MSW.

Problem

Cover the hooks with complex state and the API client layer. These are larger and need mocking (renderHook, MSW), so they come after the decomposition work and the pure-utility coverage in #1929.

Scope

Hooks (use renderHook from @testing-library/react)

File	Lines	Priority	What to cover
hooks/useTorrentActions.ts	1130	Critical	All torrent operations, optimistic updates, bulk actions
hooks/useTorrentsList.ts	278	High	Pagination, filter/search reset, cross-seed detection
hooks/useCrossSeedSearch.tsx	599	High	Search state machine, polling, cooldown
hooks/useTorrentExporter.ts	222	High	Export hook; covers dedupeTorrents, buildDownloadName, ensureUniqueFilename (may need light extraction to make internal pure helpers testable)
lib/speedUnits.ts	90	Medium	useSpeedUnits hook + pure formatSpeedWithUnit (test together — one source file)
lib/cross-seed-utils.ts	152	Medium	useLocalCrossSeedMatches hook + pure normalizePath / isInsideBase / isHardlinkManaged / toCompatibleMatch
hooks/usePersistedFilters.ts	~80	Medium	localStorage sync, per-instance filter state

useTorrentActions.ts is large enough that it should be its own PR (or multiple PRs grouped by operation type).

API (MSW)

• lib/api.ts (2696 lines): contract tests for parameter serialization, error handling, response parsing
• Set up MSW handlers mirroring the OpenAPI surface
• Start with the endpoints exercised by the hooks above

Acceptance

• Each hook above has tests for its primary state transitions and at least one error path
• MSW set up under web/src/test/ (or similar) with handlers for the covered endpoints
• make test-frontend runs and passes