Iteration 2: Bug fixes + design feedback (litmus-jvkn)

8 items: fix app theme switcher, filter navigation, remove scene sidebar, separate filters, readability score, clickable contrast issues, move CVD, richer sidebar items

Summary of Changes

All 8 items implemented:

1. Fix app theme switcher (shell.rs): Moved app_theme.read() inside use_effect so Dioxus tracks the dependency
2. Filter navigation (sidebar.rs): Added use_navigator() — search, variant, and readability filter changes push Route::ThemeList
3. Remove scene tabs from sidebar (sidebar.rs): Removed Scenes section entirely
4. Separate variant/contrast filters (sidebar.rs): All/Dark/Light now show count badges; readability is a separate dropdown below
5. Readability score (contrast.rs, state.rs, sidebar.rs, theme_list.rs, theme_detail.rs): Added readability_score(), replaced good_contrast: bool with min_readability: Option<u8>, shown on sidebar items, theme cards, and detail header
6. Clickable contrast issues (theme_detail.rs): Issues text is now a toggle button; expands to show per-scene issue list with fg/bg chips and ratios
7. Move CVD (sidebar.rs): CVD section moved below Compare, just above App Theme
8. Richer sidebar items (sidebar.rs, style.css): Items use theme bg/fg colors, show readability badge, subtle borders between items

Scene tabs moved to top of ThemeDetail (before palette, above scene content).