Finish interrupted features: tab prefetch, pet viewport, and home logo.

Add shared quiz/habits tab cache with nav hover prefetch and AppDataPrefetcher,
make the pet screen fill and center the viewport, and point the HaBit logo to /.

components/layout/habit-brand.tsx

@@ -16,7 +16,7 @@ type HabitBrandProps = {
 
 export function HabitBrand({
   className,
-  href = routes.avatar,
+  href = routes.home,
   "aria-current": ariaCurrent,
 }: HabitBrandProps) {
   const [useText, setUseText] = useState(false);