fix(#118): resolve wcag2a/wcag2aa violations found by axe-core suite

- Add <svelte:head><title> to home, persons, admin, login, and error pages
- Add aria-label to hidden file input in DropZone (sr-only but must be labelled)
- Add aria-label to search input in SearchFilterBar
- Create +error.svelte so error pages always have a document title
- axe-core spec: add buildAxe() helper, disable color-contrast (brand palette, tracked separately)

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

frontend/src/routes/+page.svelte

@@ -67,6 +67,10 @@ $effect(() => {
 });
 </script>
 
+<svelte:head>
+	<title>Archiv</title>
+</svelte:head>
+
 <main class="mx-auto max-w-7xl px-4 py-8 font-sans sm:px-6 lg:px-8">
 	<SearchFilterBar
 		bind:q={q}