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/e2e/accessibility.spec.ts

@@ -6,9 +6,9 @@ import { test, expect } from '@playwright/test';
  * Authenticated pages use the stored admin session from playwright.config.ts.
  * The login page test overrides to an unauthenticated context.
  *
- * On first run: if violations are found they are logged with full details so
- * that they can be either fixed or explicitly excluded here with a comment
- * explaining the reason.
+ * Known exclusion:
+ *   color-contrast — brand palette (ink-3, text-ink/60) does not meet AA contrast
+ *   ratios. Requires a design review with Leonie before fixing. Tracked separately.
  */
 
 const AUTHENTICATED_PAGES = [
@@ -17,13 +17,17 @@ const AUTHENTICATED_PAGES = [
 	{ name: 'admin', path: '/admin' }
 ];
 
+function buildAxe(page: Parameters<typeof AxeBuilder>[0]['page']) {
+	return new AxeBuilder({ page }).withTags(['wcag2a', 'wcag2aa']).disableRules(['color-contrast']);
+}
+
 test.describe('Accessibility — authenticated pages', () => {
 	for (const { name, path } of AUTHENTICATED_PAGES) {
 		test(`${name} page has no critical wcag2a/wcag2aa violations`, async ({ page }) => {
 			await page.goto(path);
 			await page.waitForSelector('[data-hydrated]');
 
-			const results = await new AxeBuilder({ page }).withTags(['wcag2a', 'wcag2aa']).analyze();
+			const results = await buildAxe(page).analyze();
 
 			if (results.violations.length > 0) {
 				const summary = results.violations
@@ -44,7 +48,7 @@ test.describe('Accessibility — login page', () => {
 		await page.goto('/login');
 		await expect(page.getByLabel('Benutzername')).toBeVisible();
 
-		const results = await new AxeBuilder({ page }).withTags(['wcag2a', 'wcag2aa']).analyze();
+		const results = await buildAxe(page).analyze();
 
 		if (results.violations.length > 0) {
 			const summary = results.violations