feat: add theme utility with applyTheme and isDarkMode

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>

package.json

@@ -78,6 +78,7 @@
     "@vite-pwa/sveltekit": "^1.1.0",
     "@vitest/coverage-v8": "^3.2.4",
     "drizzle-kit": "^0.31.9",
+    "jsdom": "^29.0.1",
     "nyc": "^18.0.0",
     "svelte": "^5.53.7",
     "svelte-check": "^4.4.4",