Dark theme: Visual audit on all screens

[sethusrinivasan]

Problem

Dark theme toggle works but some screens may have hardcoded colors that don't adapt.

Done

• DocumentViewerScreen cards → MaterialTheme.colorScheme.surface/surfaceVariant
• Boundary message → surfaceVariant
• Color.Gray for secondary text → acceptable in both themes

TODO

• Visual test every screen in dark mode
• Check tag color chips readability on dark background
• Check EULA text contrast
• Check Import screen, Backup screen, Review screen