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/DropZone.svelte

@@ -202,6 +202,7 @@ $effect(() => {
 	type="file"
 	multiple
 	accept=".pdf,.jpg,.jpeg,.png,.tif,.tiff"
+	aria-label={m.upload_label()}
 	class="sr-only"
 	onchange={handleFileSelect}
 />