ui(theme): fix invisible swatch border in dark themes and add mono fallback to Theme Studio #1654

Problem

1. In lib/features/settings/theme_preview_card.dart (_Swatch, lines 225–227):

   border: material.Border.all(
     color: material.Colors.black.withValues(alpha: 0.12),
   ),

   A semi-transparent black border is used around color swatches. In dark themes (Dark, Tokyo Night, Monokai, Dracula), dark swatch colors (such as background #1e1e1e or surface #18181b) have zero border contrast against dark popover backgrounds, making swatch boundaries invisible.
2. In lib/features/settings/theme_editor_dialog.dart (_ThemeEditorColorRow, line 346):

   style: material.TextStyle(
     fontSize: 12,
     color: cs.mutedForeground,
     fontFamily: QueryaTypography.mono,
   ),

   The hex code displays with QueryaTypography.mono but omits fontFamilyFallback: QueryaTypography.monoFontFamilyFallback.

Solution

• Use Theme.of(context).colorScheme.border or context.workbench.borderSubtle for swatch borders so they adapt properly in both light and dark themes.
• Provide fontFamilyFallback: QueryaTypography.monoFontFamilyFallback in _ThemeEditorColorRow.