[codex] update empty state icon

[ichliebewxy]

What changed

• Replaced the empty chat compass icon with the provided custom image asset.
• Added sizing, object-fit, rounded corners, and a subtle shadow for the new image icon.
• Preserved the current brand-mark text update in frontend/index.html as part of the requested full upload.

Why

The chat empty state should use the new supplied visual instead of the Font Awesome compass icon.

Validation

• Confirmed the image asset loads locally and renders at 72x72.
• Ran git diff --cached --check successfully before committing.