Update Welcome panel assets and styling

[huntie]

Summary

Updates to the appearance of the Welcome panel:

• Update product icon to match incoming desktop app.
• Update "Learn" assets for iOS/macOS 26.
• Tweak colours to fit latest Chrome DevTools appearance, better dark mode matching.
• Tweak some sizes and borders.
• Improve styling for FB-only session ID box.

Test plan

Before: https://pxl.cl/8d3Vv
After: https://pxl.cl/8d3VD
After (dark appearance): https://pxl.cl/8d3VJ

• This change maintains backwards compatibility with previous Local Storage data (if modifying settings, experiments, or other persisted client state).

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo following the contribution guide for Meta employees OR contribution guide.
• This commit is React Native-specific and cannot be upstreamed.