feat(ui): complete config page overhaul with sidebar nav, search, toggles, and diff view

Major redesign of the config page:

Layout:
- Sidebar navigation with section list
- Search input to filter settings
- Section cards with icons and descriptions
- Responsive design for mobile (stacked layout)

Fields:
- New toggle switches for booleans (replaces checkboxes)
- Improved field-row layout with label, help text, and control
- Better fieldset and array styling

Features:
- Diff view showing pending changes before save
- Original value tracking for comparison
- Section filtering via sidebar nav
- Search across setting names, descriptions, and nested properties

Styling:
- Dedicated config.css with all new styles
- Dark and light theme support
- Smooth animations and transitions
- Mobile-first responsive breakpoints

ui/src/styles.css

@@ -2,3 +2,4 @@
 @import "./styles/layout.css";
 @import "./styles/layout.mobile.css";
 @import "./styles/components.css";
+@import "./styles/config.css";