refactor: replace shallow SWR hooks with domain modules (Phase 5)

[ivannissimrch]

Problem

20+ individual hooks each wire their own loading/error/mutation plumbing. No shared cache key ownership. Components manage hook complexity instead of just rendering.

Solution

Replace with domain modules:

• hooks/lessons/useLessons.ts — exports useLesson(id), useLessons(), useCreateLesson(), useUpdateLesson(), useDeleteLesson()
• hooks/assignments/useAssignments.ts
• hooks/users/useUsers.ts
• hooks/lists/useLists.ts
• hooks/practiceWords/usePracticeWords.ts

Each module owns its cache keys and invalidation strategy.

Commits

28: Move useAddLesson + useEditLesson into hooks/lessons/, update imports
29: Create useLessons.ts domain module, migrate components one at a time
30: Delete old individual lesson hooks
31-35: Repeat for assignments, users, lists, practiceWords domains

Part of architecture roadmap

#292 — Phase 5