feat: filter viewed code review files

[MattPua]

Problem

Code reviews with many files make it hard to focus on changes that still need review.

Changes

• Add a Hide viewed files option to diff settings
• Combine viewed-state filtering with comment filters
• Preserve section headers and clear filters for direct file navigation

How did you test this?

• Ran the @posthog/ui test suite: 2,115 tests passed
• Ran Biome checks on all changed files

Automatic notifications

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

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Created with PostHog Code

[trunk-io]

😎 Merged directly without going through the merge queue, as the queue was empty and the PR was up to date with the target branch - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 098c8a7.}

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

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### Issue 1 of 1
packages/ui/src/features/code-review/components/ReviewShell.tsx:180-187
**Active file remains hidden**

When “Hide viewed files” is enabled and the active file is marked viewed, this filtering removes its diff and navigation-index entry without updating the active path, causing the Changes panel to keep highlighting a file that is no longer displayed until the user scrolls or navigates elsewhere.

_{Reviews (1): Last reviewed commit: "feat: filter viewed code review files" | Re-trigger Greptile}

[github-actions]

Contained UI feature adding a "hide viewed files" filter to the code review panel, with new tests covering the filter and active-path-resolution logic; the added useEffect in ReviewShell.tsx directly resolves the sole reviewer (Greptile) concern about the active file staying hidden after filtering. No risky territory touched.

• Author wrote 100% of the modified lines and has 10 merged PRs in these paths (familiarity STRONG).
• 👍 on the PR from greptile-apps[bot].

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✓	no deny categories matched
size	✓	154L, 5F substantive, 217L/7F incl. docs/generated/snapshots — within ceiling
tier	✓	T1-agent / T1c-medium (217L, 7F, single-area, feat)
stamphog 2.0.0b3		.stamphog/policy.yml @ af8d1ce · reviewed head 7dc619d