fix(auth): lift the register submit button and add a branded not-found page

Promote the register page's submit button to the 44 px tap-target
floor (size="lg" + min-h-11 + w-full) so the primary action stays
finger-tap reachable on a narrow viewport.

Add a new branded not-found.tsx page. The previous implicit fallback
was Next.js's bare framework text; this introduces a lighter wrapper
than ErrorDetails (404 is not an application error) — Logo, headline,
explanatory paragraph, and a single "Back to dashboard" link. The
wrapper carries min-h-dvh and safe-area-inset-top so the panel
anchors centred under iOS Safari's animated URL bar and stays clear
of the notch.

Apply the same dvh + safe-area pattern on the root-layout global
error boundary and lift its inline button heights to 44 px. Wrap the
ErrorDetails panel in a min-h-dvh flex column so server-error pages
also centre instead of hugging the notch.

src/components/error-details.tsx

@@ -74,7 +74,12 @@ export function ErrorDetails({
   }
 
   return (
-    <div className="mx-auto w-full max-w-xl space-y-4 p-6">
+    // v1.4.27 MB6 — `min-h-dvh flex flex-col justify-center` makes
+    // the error panel anchor in the middle of the available viewport
+    // even when this component is mounted at the root layout. Without
+    // it the panel hugged the top edge under the notch on
+    // small-screen devices.
+    <div className="mx-auto flex min-h-dvh w-full max-w-xl flex-col justify-center space-y-4 p-6">
       <h1 className="text-xl font-semibold">{t("errorBoundary.title")}</h1>
       <p className="text-muted-foreground text-sm">
         {t("errorBoundary.description")}