Add support for a persistent "custom" theme that users can edit and preview in settings.
- add CustomThemeColors type and storage helpers (getCustomThemeColors, setCustomThemeColors)
- apply custom CSS variables at runtime and provide applyCustomColors to live-update variables
- include fallback CSS for html.custom to avoid flash before hydration
- add ColorPicker component with curated swatches for each color role
- extend SettingsModal to support editing, previewing, and persisting custom theme colors; exclude 'custom' from regular theme lists and read custom colors when rendering previews
- tighten theme detection logic to treat custom background luminance as light/dark when needed
This enables users to define, persist, and see live updates for custom color palettes.
Add five new light theme options (lavender, rose, sand, sky, slate) with dedicated color palettes. Redesign Settings modal with sidebar navigation replacing tabs, add visual theme color selector showing preview swatches, and introduce new icons (KeyIcon, PaletteIcon, UserIcon, DocumentIcon). Also update PrivacyModal to render conditionally based on authEnabled and add privacy policy link. Update tests to reflect new UI structure.
