Aligns the "Data source" list items in the side panel by resolving a
browser-specific layout discrepancy where icons with different intrinsic
widths (e.g., "edit" vs "calendar") caused uneven text indentation and
vertical misalignment.
- Refactored `SourceHead` in `head.tsx` to wrap all list icons in a
`list-icon-wrapper` container instead of using `<List.Icon>` directly.
- Added CSS rules in `index.css` to format `.list-icon-wrapper` and its
sibling `.content` as table cells aligned at the top, with a fixed
width of `20px` for the icon column.
- Converted relative sizing (`em`) to absolute pixels (`px`) for
pixel-perfect cross-browser precision.
- Updated Playwright visual regression snapshots to match the newly
aligned and corrected UI layout.
