fix(spaces): rebrand section and row icons

[puemos]

Problem

The Spaces sidebar lost its distinct section glyphs and previous disclosure behavior, making section state and space identity less clear.

Changes

• Restore Starred and Spaces section icons, swapping to up/down carets on hover or keyboard focus.
• Use cube glyphs for public spaces across sidebar, tabs, breadcrumbs, and related controls while preserving legacy channel hashes and the private-space lock.

Why: Keep the new Spaces layout visually distinct while retaining familiar section interactions.

Show me !!

How did you test this?

• pnpm --filter @posthog/ui exec vitest run src/features/canvas/components/channelGlyph.test.tsx src/features/canvas/components/ChannelsList.test.tsx
• pnpm --filter @posthog/ui typecheck
• pnpm exec biome check on all changed files

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 15a4d56.}