ui: Refactor isMobile as reactive value in viewport store (#23330)

* refactor: `isMobile` as reactive value in `viewport` store

* refactor: Use Svelte media query for the viewport store

tools/ui/src/lib/hooks/is-mobile.svelte.ts

@@ -1,8 +0,0 @@
-import { DEFAULT_MOBILE_BREAKPOINT } from '$lib/constants';
-import { MediaQuery } from 'svelte/reactivity';
-
-export class IsMobile extends MediaQuery {
-	constructor(breakpoint: number = DEFAULT_MOBILE_BREAKPOINT) {
-		super(`max-width: ${breakpoint - 1}px`);
-	}
-}