feat: group review files by type

[MattPua]

Problem

Expanded code reviews only organize changed files by folder structure, which makes it harder to review implementation, tests, documentation, and generated files together.

Changes

• Add a Folders / File type selector to the expanded review sidebar
• Group files into implementation, tests, generated, documentation, configuration, assets, and other
• Show full paths in type groups and detect common generated and test filename patterns
• Add focused classification and ordering tests

Screenshots: not included for this compact sidebar control.

How did you test this?

• pnpm --dir packages/ui exec vitest run src/features/task-detail/components/ChangesTreeView.test.ts
• pnpm --filter @posthog/ui typecheck
• pnpm exec biome lint packages/ui/src/features/task-detail/components/ChangesPanel.tsx packages/ui/src/features/task-detail/components/ChangesTreeView.tsx packages/ui/src/features/task-detail/components/ChangesTreeView.test.ts packages/ui/src/features/task-detail/components/changesTree.ts packages/ui/src/primitives/TreeDirectoryRow.tsx
• Commit hooks ran pnpm typecheck

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 89c5712.}

[github-actions]

This PR has had no activity for 7 days and has been marked stale. We are moving to the monorepo and tightening PR staleness in preparation, so it will be closed in 7 days if no further activity occurs.