feat(web): React activity views — sessions + projects (AUR-269)

[mishanefedov]

Linear: AUR-269

Summary

Visualizes the per-event activity classifier (AUR-264) data through two new lazy routes.

`/sessions/:id/activity`

• Stacked bar of events × category in 1-min UTC buckets across the session's lifetime.
• Per-category summary table (events, cost, % of events, % of cost).
• Bucketed client-side from the session's events (which already carry `details.category`) so this didn't need a new time-series API.
• Empty state for sessions with zero classified events.

`/projects/:name/activity`

• Pie of events by category + pie of cost by category side-by-side.
• Per-category breakdown table including the new sessions touched column.
• Empty state for unclassified projects.

API extension

`activityByProject` now adds `COUNT(DISTINCT session_id) AS sessions_touched`. `ActivityBucket` interface gets an optional `sessionsTouched` field. The web client's `EventDetails` type also exposes the existing `details.category` field.

Wiring

• Both pages are lazy-loaded (`Suspense` fallback), matching `SessionTokens`, `Trends`, `SessionGraph`.
• Nav links from `Session.tsx` and `ProjectDetail.tsx`.

Acceptance criteria

• New route `/sessions/:id/activity` rendering recharts stacked-bar over time + per-category cost summary.
• New route `/projects/:name/activity` with pie + table including `events / cost / sessions touched`.
• Both views handle the empty-data case gracefully.
• Lazy-loaded in `main.tsx`.

Test plan

• `npm run typecheck` — clean (server)
• `web && npx tsc --noEmit` — clean for new files (one pre-existing unused-var error in `Logs.tsx` left alone)
• `npm test` — 369/369 pass (+1 new for `sessionsTouched`)
• `npm run build` — server (tsup) + web (vite) both build