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/admin/+page.svelte

@@ -11,6 +11,10 @@ let { data, form } = $props();
 let activeTab = $state('users');
 </script>
 
+<svelte:head>
+	<title>Administration</title>
+</svelte:head>
+
 <div class="mx-auto max-w-7xl px-4 py-8 font-sans sm:px-6 lg:px-8">
 	<div class="mb-8 flex flex-col gap-4 sm:flex-row sm:items-center sm:justify-between">
 		<h1 class="font-serif text-3xl text-ink">{m.admin_heading()}</h1>