feat(a11y): add aria-describedby to Weiter button when aria-disabled

Screen readers now announce the hint paragraph text on focus when no type
is selected, so users hear why the button is disabled without having to
click it first.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/geschichten/new/TypeSelector.svelte

@@ -77,7 +77,7 @@ const descs: Record<GeschichteType, () => string> = {
 	<div aria-live="polite" aria-atomic="true" class="sr-only">{announcement}</div>
 
 	{#if !selected}
-		<p class="mt-3 font-sans text-xs text-ink-3" aria-hidden="true">
+		<p id="type-hint" class="mt-3 font-sans text-xs text-ink-3" aria-hidden="true">
 			{m.journey_selector_aria_live_hint()}
 		</p>
 	{/if}
@@ -85,6 +85,7 @@ const descs: Record<GeschichteType, () => string> = {
 	<button
 		type="button"
 		aria-disabled={selected == null ? 'true' : 'false'}
+		aria-describedby={selected == null ? 'type-hint' : undefined}
 		tabindex="0"
 		onclick={handleWeiter}
 		class="mt-6 inline-flex h-11 items-center rounded px-6 font-sans text-sm font-medium transition-opacity focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring {selected == null