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/+error.svelte

@@ -0,0 +1,12 @@
+<script lang="ts">
+import { page } from '$app/state';
+</script>
+
+<svelte:head>
+	<title>Fehler – Familienarchiv</title>
+</svelte:head>
+
+<div class="px-4 py-12 text-center font-sans">
+	<p class="font-sans text-6xl font-bold text-ink">{page.status}</p>
+	<p class="mt-2 font-sans text-sm text-ink-2">{page.error?.message ?? 'Internal Error'}</p>
+</div>