Add Slack DM toggle for channel thread @-mentions

[adboio]

Problem

@-mentions in channel threads only surface in the polled Activity feed — if you're not in the app, you don't find out someone tagged you. PostHog/posthog#72087 adds backend-driven Slack DMs on mention (opt-in, via the team's existing Slack integration); this PR adds the client-side toggle for that preference.

Changes

• New SlackMentionNotificationsSettings section rendered in Slack settings below the inbox-notifications block: a "Mention notifications" switch ("Get a Slack DM when someone @-mentions you in a channel thread"). Hidden unless the project-bluebird flag is on and a Slack workspace is connected (the parent section owns the connect prompt).
• useCodeUserNotificationSettings / ...Mutations hooks (query + optimistic-update mutation with revert-and-toast on failure), following the useSignalUserAutonomyMutations pattern.
• api-client: getCodeUserNotificationSettings / updateCodeUserNotificationSettings against the new /api/code/user_settings/ endpoint, with the CodeUserNotificationSettings type in @posthog/shared/domain-types.
• Toggle is instrumented with SETTING_CHANGED (setting_name: "slack_mention_notifications").

Depends on PostHog/posthog#72087 — until that deploys, the settings fetch 404s and the section shows the switch in its default (off) state.

How did you test this?

Code review against the sibling Slack settings sections only — I (Claude, via a PostHog Code cloud task) could not run pnpm typecheck/pnpm lint or the app in this sandbox (no installed workspace), so CI is the verification here. No new unit tests: the hooks mirror useSignalUserAutonomyMutations and the component is presentational; sibling settings sections carry no component tests either.

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 029c2b5.}