[codex] Simplify Tab Out UI

[Burt-code]

What changed

• Removed the greeting/date header so the Open tabs section starts at the top.
• Removed the fixed footer with the Tab Out attribution and open-tab count.
• Simplified Saved for later into a saved-list manager by removing checkboxes and the archive section.
• Refreshes the open-tab groups after saving a tab for later so an empty domain group disappears immediately.

Why

The preview had redundant chrome around the main workflow. Saved for later should manage temporarily saved pages only, while browser bookmarking/archiving belongs to Chrome itself.

Validation

• node --check extension/app.js
• git diff --check