Move Channels switch into the task panel header

[k11kirky]

Problem

The Channels mode control occupies a global navigation row even though it changes the contents of the task panel. Moving the control into that panel makes its scope clearer and keeps mode-specific actions beside the content they affect.

Changes

mode.switcher.mov

• Replace the Channels navigation row with a compact Channels/List selector in the panel header
• Show task search, folder, and filter actions only while viewing Tasks
• Keep Activity directly below Inbox whenever the Channels feature flag is enabled
• Hide the selector and Activity when the Channels feature flag is disabled

How did you test this?

• pnpm --filter @posthog/ui typecheck
• pnpm --dir packages/ui exec vitest run src/features/sidebar/components/SidebarNavSection.test.tsx src/features/sidebar/components/TasksHeader.test.tsx src/features/sidebar/components/CustomizeSidebarDialog.test.tsx
• pnpm exec biome check packages/ui/src/features/canvas/components/ChannelsSidebar.tsx packages/ui/src/features/sidebar

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 44efe8f.}

[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.

---

### Issue 1 of 1
packages/ui/src/features/sidebar/constants.ts:6-12
**Persisted orders bypass Activity placement**

When an existing user has a non-empty persisted `navItemOrder`, `orderedNavItems` retains Activity's stored position instead of applying this new default, causing Activity to remain after Command Center or elsewhere rather than directly below Inbox.

_{Reviews (1): Last reviewed commit: "Increase sidebar mode label size" | Re-trigger Greptile}

[github-actions]

Contained UI-only sidebar refactor with no risky-territory surface touched; the one substantive reviewer concern (Activity ordering with persisted nav state) is directly fixed in constants.ts with new test coverage, and a teammate approved the current head.

• Author wrote 5% of the modified lines and has 18 merged PRs in these paths (familiarity MODERATE).
• adamleithp reviewed the current head.

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✓	no deny categories matched
size	✓	196L, 7F substantive, 376L/11F incl. docs/generated/snapshots — within ceiling
tier	✓	T1-agent / T1d-complex (376L, 11F, single-area, unknown)
stamphog 2.0.0b3		.stamphog/policy.yml @ cee8b0e · reviewed head 44efe8f

[k11kirky]

/trunk merge