Fix Home navigation from the editor: derive the landing deep-link redirect from the router url

[milanofthe]

The landing page computed redirecting once at component init from window.location.search. During a client-side navigation from /editor back to /, the component initializes before the history entry updates, so the check still saw the editor's query (e.g. ?model=... from an example deep link). redirecting came out spuriously true: the landing rendered empty and immediately bounced back to the editor via replaceState. Net effect: the Home button set the URL but never navigated.

Fix: derive redirecting from page.url.searchParams ($app/state), and move the redirect from a one-shot onMount into an $effect reacting to the same source. Direct deep-link loads (/?model=..., ?backend=...) behave exactly as before.

Repro before the fix: landing -> click an example (lands on /editor?model=...) -> click the Home button in the nav bar.