Replace useProjectsCache with Tanstack Query (or SWR)

## Context

Per review comment on #184 by @svozza: the hand-rolled `useProjectsCache` hook (~200 lines) manually implements stale-while-revalidate, dedup, TTL, and error states. A library like [Tanstack Query](https://tanstack.com/query) or [SWR](https://swr.vercel.app/) provides all of this out of the box with devtools, retry, prefetch, and garbage collection.

## Expected outcome

- Replace `useProjectsCache.ts` with Tanstack Query hooks
- Reduce cache boilerplate by ~50%
- All consumers (`AppSidebar`, `Dashboard`, `Project`, `ObservabilityLayout`) updated
- Behavior unchanged: stale-while-revalidate, 120s TTL projects, 60s sprints

## Notes

- Tanstack Query is tree-shakeable and ~12kB gzipped
- This is a refactor, no user-facing behavior change

Start by reading useProjectsCache.ts and tracing its consumers: AppSidebar, Dashboard, Project, and ObservabilityLayout. Compare Tanstack Query and SWR against the existing stale-while-revalidate, deduplication, error, and TTL behavior. Done means the consumers use the selected library, the cache hook is replaced, boilerplate is reduced, and projects and sprints retain their 120-second and 60-second TTLs.

Written by the indexing model from the issue text.