Adds src/lib/utils/theme.svelte.ts with applyTheme() (sets data-theme
attribute and meta theme-color) and getIsDarkMode(). Uses a plain
variable instead of $state since Vitest's node environment does not
compile Svelte 5 runes in .svelte.ts files. Adds jsdom dev dependency
and $app/environment alias in vitest config to support DOM-level unit
tests; fixes preferences.test.ts to include the theme default added
in Task 2.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
