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.
