feat(channels): add thread @-mentions and an Activity page

[k11kirky]

Note

Stacked on #3157 (channel/thread deep links) — review that first; this PR targets its branch and reuses its thread deep-link routing.
The mentions feed is served by a new backend index — PostHog/posthog#68853 must deploy first. Until it does, the Activity page shows an empty state (acceptable for a pre-GA surface behind the project-bluebird flag).

Problem

Channel threads (project-bluebird) are multiplayer, but there's no way to pull a teammate into one — you can't @-tag anyone, and nothing tells you a thread needs your attention.

Why

Requested for Bluebird channels: tag any member of your org in a thread and have them see the notification in a new Activity page in the top-left nav, built on the deep links from #3157.

Changes

• @-mentions in thread composers — typing @ in a thread's "Reply in thread…" box opens a typeahead over the org's members (new /api/organizations/@current/members/ client call). Selecting one inserts an inline @[Name](email) token, so mentions ride in existing message content; older clients degrade to readable text. Mentions render as highlighted chips, with a stronger highlight when it's you.
• Activity page — new /website/activity route plus an Activity item (with unread red-dot badge) in the channels sidebar nav. It lists thread messages across all channels that mention you, newest first; clicking one opens the thread through the channel deep-link route from feat(channels): add shareable deep links for channels and threads #3157, and each row offers the shareable thread link via the same copy-link helper.
• The feed comes from the backend mentions index (PostHog/posthog#68853): useMentionActivity wraps exactly one getTaskMentions() query against the new GET /task_mentions/ endpoint (AGENTS.md Rule 5). An earlier iteration on this branch assembled the feed client-side by scanning every channel's task feeds and threads; that fan-out and its mention-matching helpers are deleted, and mentionActivity in core is reduced to a DTO→item mapper. ActivityView resolves desktop folder-channel ids itself (same caller-owns-the-subscription precedent as useTaskChannelMap).
• Unread state is a persisted last-seen timestamp; opening the page clears the badge.
• Instrumented via Channel action (mention_member, view_activity, open_mention, nav_click). Everything lives inside the Channels space, which is gated by the project-bluebird flag at the route.

How did you test this?

• Unit tests: mention token parse/format (shared), DTO→item mapping + unseen counting (core), composer caret/query/insert helpers (ui), getTaskMentions client shape (api-client).
• Full suites pass: ui (1245), core (1991), shared (441), api-client (66).
• Full-repo pnpm typecheck (22/22), Biome clean, knip shows no new dead exports.
• Not run: end-to-end in the packaged app (no desktop environment in this session), and the endpoint itself wasn't exercised against a live backend — the server half must land and deploy first.

Automatic notifications

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

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Created with PostHog Code

[github-actions]

React Doctor found 1 issue in 1 file · 1 warning.

1 warning

src/features/canvas/components/MentionComposer.tsx

• ⚠️ src/features/canvas/components/MentionComposer.tsx:142 Role used instead of HTML tag prefer-tag-over-role

_{Reviewed by React Doctor for commit 8c7055c.}

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