feat(activity): add mark-as-read actions

[puemos]

Problem

Activity items are only marked read as a side effect of opening them, leaving no way to clear an item without navigating away or clear the feed at once.

Why: Give people direct control over their Activity unread state.

Show me!!

Changes

• Add a row-level mark-as-read action for unread activity
• Add a header action to mark all displayed unread activity as read
• Reuse the existing optimistic mutation and show pending state for bulk updates

How did you test this?

• pnpm exec biome check packages/ui/src/features/canvas/components/ActivityView.tsx
• pnpm --filter @posthog/ui typecheck
• pnpm --filter @posthog/ui test -- ActivityView.test.tsx useTaskActivity.test.tsx

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 1 issue in 1 file · 1 warning.

1 warning

src/features/canvas/components/ActivityView.tsx

• ⚠️ src/features/canvas/components/ActivityView.tsx:246 Chained array iterations js-combine-iterations

_{Reviewed by React Doctor for commit 2d01dea.}