fix: support Slack workspaces in loop notifications

[MattPua]

Problem

Loop Slack destinations searched only the first connected workspace, making channels in other connected workspaces unavailable.

Changes

Before:

After

CleanShot.2026-07-22.at.12.37.08.mp4

• add a shared Slack workspace selector
• select a workspace before choosing a Loop notification channel
• preload the first channel page and keep search server-side
• preserve accessible loading and clearing behavior

How did you test this?

• pnpm --filter @posthog/ui typecheck
• pnpm exec vitest run src/features/settings/components/SlackChannelCombobox.test.tsx
• Biome check on changed files
• fresh QA reviewer pass

Root pnpm typecheck remains blocked by unrelated existing @posthog/web module-resolution errors for @posthog/agent and @posthog/host-trpc.

Automatic notifications

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

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Created with PostHog Code

[trunk-io]

😎 Merged successfully - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 84328e8.}

[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/settings/sections/SlackInboxNotificationsSettings.tsx:68
**Team Default Uses First Workspace**

With multiple Slack integrations, this always loads the team default channel from `slackIntegrations[0]`. If the saved default belongs to another workspace, the picker queries the wrong channel list and offers no way to select the correct workspace, so the displayed workspace and persisted channel can disagree.

_{Reviews (1): Last reviewed commit: "refactor: extract Slack workspace channe..." | Re-trigger Greptile}