Replace slate-blue UI with charcoal theme

[marcmy]

Summary

• replace CompactGUI's slate-blue application backdrop with a neutral charcoal surface using a broad, low-contrast radial glow rather than a conspicuous full-height gradient
• use ScRGB interpolation plus a deterministic one-level dither overlay to break up WPF gradient banding/streaks without adding visible texture
• keep muted slate/light-blue presentation colors neutral gray while preserving semantic/status colors
• make checked checkbox glyphs and borders use the CompactGUI blue (#4CA5FF)
• give the selected compression-mode card a persistent 2px blue outline and directly enforce that outline on WPF-UI's rendered card border so template/style precedence cannot swallow it
• include the latest Start CompactGUI UI overhaul foundation #26 navigation/details polish because this remains the complete combined test branch

Background refinement

The previous radial pass looked better overall but still quantized into visible broad streaks. This revision lowers the radial contrast, stretches the falloff farther beyond the viewport, uses ScRgbLinearInterpolation, and overlays deterministic 1-alpha black/white dither. The dither is intended to be visually imperceptible while preventing large same-shade contours from forming.

Selection visibility

The XAML checked trigger remains, but the custom CompressionMode_Radio now also applies the selected border directly to both the WPF-UI Card and its rendered ContentBorder on load, template application, Checked, and Unchecked. Selected is #4CA5FF at 2px; unselected remains a subtle 1px neutral outline.

Stack / combined test branch

This PR is stacked on #26, which is stacked on #28, which is stacked on #24. agent/ui-charcoal-theme is the complete test branch for the manual resume/exit fixes, corrected non-compressible-file filtering, UI-overhaul foundation, and this charcoal theme.

Relative to #26, this PR changes:

• CompactGUI/MainWindow.xaml.vb
• CompactGUI/Services/CharcoalTheme.vb
• CompactGUI/Views/Pages/PendingCompression.xaml.vb
• CompactGUI/Views/Components/CompressionMode_Radio.xaml
• CompactGUI/Views/Components/CompressionMode_Radio.xaml.vb

Validation

The exact combined source tree was temporarily validated against master and then the branch was restored to its stacked base.

• Windows x64 restore: passed
• self-contained publish: passed
• packaging: passed
• artifact upload: passed
• supply-chain audit: passed

Runtime checks

1. Verify the background still has gentle charcoal depth but no obvious contour/streak bands.
2. Verify the selected compression mode always has a clear 2px blue outline, including immediately on load and after clicking/hovering other cards.
3. Verify checked configuration boxes retain the blue checkmark/border.
4. Re-run the prior Compress icon, Details scrollbar/column-separator, save-progress, Resume-idle, and skip-filtering checks on the combined branch.