perf(web): avoid per-row sidebar project lookups

[ronak-guliani]

Summary

• Removes the per-row useStore(selectProjectByRef(...)) lookup from Sidebar thread rows.
• Reuses the project group's existing scoped-project map to pass each row's owning project cwd directly.
• Keeps git cwd resolution behavior unchanged: worktree path, then owning thread project cwd, then displayed project cwd fallback.

Performance impact

This targets the next Sidebar hot path from the audit: visible thread rows no longer create one store subscription and selector per row just to resolve the project cwd used for git-status/PR detection. The project-level memoized map is built once and reused while rendering rows.

Validation

• cd apps/web && bun run test src/components/Sidebar.logic.test.ts
• bun fmt apps/web/src/components/Sidebar.tsx apps/web/src/components/Sidebar.logic.ts apps/web/src/components/Sidebar.logic.test.ts
• bun lint

Note: bun typecheck currently fails on the origin/main baseline in unrelated files (ChatView.browser.tsx, environmentApi.ts, store.ts); the Sidebar changes themselves are covered by the focused test above.