fix(webui): polish dialogs for theme legibility

Source picker
- Light-mode source-card body, icon tile, and pale Unknown/AOSP
  icons now have visible contrast on white backgrounds.
- Center the orphan Custom keybox card via grid-column span when
  the row count is odd.
- Soften the bottom-fade gradient in light mode so the FETCH
  INTERVAL caption stays legible.

Mode picker
- Add class="mode-auto" to the default radio so Auto no longer
  inherits the global accent.
- Shift Generated from green (#36DB2B) to purple (#A855F7) to
  break the green-on-green collision with Auto's teal.
- Per-row color stripes plus tinted backgrounds keep the three
  modes distinguishable on any accent or theme.

Radii
- Bump .source-icon (12->14), .update-card (8->12), and
  .system-app-item (0->10) for consistent rounding.

Author: Enginex0