feat(channels): add the flag-gated spaces layout (3/4)

[puemos]

Problem

Tasks and canvases need a shared channel home with navigation that stays scoped to the active channel.

Why

Switching channels through a popover buried the channel list behind a click and gave it nowhere to grow. The list is the main way around the space, so it gets a pane of its own.

Changes

• Add the complete channel sidebar, navigation, creation controls, and starred-channel shortcuts.
• The sidebar is a master/detail slider: a searchable channel list (#me, starred, then the rest) that slides aside when you open a channel, and a channel pane headed by a row that slides back. Browsing the list doesn't leave the channel you're in.
• Integrate the spaces chrome, settings visibility, sizing, error fallback, and analytics.
• Activate the experience behind code-spaces-layout plus project-bluebird.
• Keep the existing Thread panel in this intermediate state.

This is PR 3 of the stack, based on #3813. Flag off remains the existing experience; flag on becomes Spaces with the legacy Thread panel.

How did you test this?

• pnpm --filter @posthog/core test -- src/command-center/grid.test.ts
• Focused UI suites covering flag transitions, shortcuts, the sidebar slider, channel search, sidebar behavior, settings, and glyphs
• pnpm --filter @posthog/ui test (249 files, 2184 tests) and pnpm --filter @posthog/ui typecheck

Automatic notifications

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

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Created with PostHog Code

[github-actions]

React Doctor found 3 issues in 3 files · 3 warnings.

3 warnings

src/features/canvas/components/ChannelItemRow.tsx

• ⚠️ src/features/canvas/components/ChannelItemRow.tsx:58 Role used instead of HTML tag prefer-tag-over-role

src/features/canvas/components/ChannelNav.tsx

• ⚠️ src/features/canvas/components/ChannelNav.tsx:75 Pure function rebuilt every render prefer-module-scope-pure-function

src/features/sidebar/components/ProjectSwitcher.tsx

• ⚠️ src/features/sidebar/components/ProjectSwitcher.tsx:66 Large component is hard to read and change no-giant-component

_{Reviewed by React Doctor for commit f9fdbf8.}

[puemos]

• feat(channels): add the structured Activity view (4/4) #3815
• feat(channels): add the flag-gated spaces layout (3/4) #3814 👈 (View in Graphite)
• feat(channels): add scoped channel foundations (2/4) #3813
• refactor(canvas): share thread and PR artifact logic (1/4) #3812
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[posthog]

✅ Visual changes approved by @puemos — baseline updated in 8a9e6f2.

View this run in PostHog

12 new, 12 removed.

[puemos]

Merge activity

• Jul 28, 8:32 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Jul 28, 8:52 AM UTC: Graphite couldn't merge this pull request because a downstack PR refactor(canvas): share thread and PR artifact logic (1/4) #3812 failed to merge.

[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.