- Add Project model with owner-based access and cascade delete
- Restructure Category to belong to either Home (userId) or a Project (projectId), enforced by DB constraints and access filters
- Scope all board operations (create, update, delete, reorder) to the correct board context using `categoryAccessFilter` and `projectAccessFilter`
- Replace hardcoded `revalidatePath("/")` with dynamic `boardPath()` for proper cache invalidation
- Add `ProjectsProvider` and `ProjectsNavSection` for project sidebar navigation
- Make KanbanBoard accept `projectId` and `title` props to render Home or Project boards
- Extract `getBoard()` utility to centralize board data fetching
- Refactor group/todo/notes actions to derive board context from category membership rather than trusting caller input
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