fix(ui): enforce WCAG AA contrast thresholds across UI themes

- Raise accessible_accent default threshold from 2.0 to 3.0 (WCAG 1.4.11 UI state indicators)
- Raise EntryKind::Info command line history threshold from 3.0 to 4.5 (WCAG 1.4.3 normal text)
- Update themed_check_cell to verify >= 3.0:1 contrast across both weak popup panel and strong highlighted rows
- Replace fully-qualified accessible_accent_threshold calls inside closures with use imports
- Add comprehensive 9-suite theme accessibility contrast test suite covering all 22 themes across core surfaces

src/ui/mod.rs

@@ -25,3 +25,6 @@ pub use properties::PropertiesPanel;
 pub use ribbon::Ribbon;
 pub use statusbar::StatusBar;
 pub use window::layers::LayerPanel;
+
+#[cfg(test)]
+mod theme_accessibility_tests;