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/login/+page.svelte

@@ -9,6 +9,10 @@ const localeMap = { DE: 'de', EN: 'en', ES: 'es' } as const;
 const activeLocale = $derived(getLocale().toUpperCase());
 </script>
 
+<svelte:head>
+	<title>Anmelden</title>
+</svelte:head>
+
 <div class="relative flex min-h-screen flex-col bg-canvas">
 	<!-- Language switcher -->
 	<div class="absolute top-4 right-4 flex items-center gap-1">