Replace useProjectsCache with Tanstack Query (or SWR)

[eipasteur]

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 or SWR 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