Keep channel day dividers sticky

[klopez4212]

Summary

• Restore sticky day labels in channel timelines while letting the divider line scroll with the message group.
• Keep day pills above the timeline content but behind the composer/header chrome.
• Remove the pill shadow while preserving the solid background.

Snapshot

Checks

• git diff --check HEAD
• pnpm --dir desktop check
• pnpm --dir desktop typecheck
• Combined validation before splitting: pnpm --dir desktop test, pnpm --dir desktop build, focused Playwright smoke/integration suites

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: af8c90dcc1

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".