fix(#71-#73): address all review findings from Markus and Sara

BLOCKERs:
- Remove direct AppUserRepository/CommentRepository access from CommentService and
  NotificationService — replaced with UserService.findAllById() and UserService
  (fixes layering contract from CLAUDE.md)
- Switch Optional<JavaMailSender> constructor injection — removes @Autowired(required=false)
  field and ReflectionTestUtils hack in tests
- Add @RequirePermission(READ_ALL) to UserSearchController — prevents user enumeration
  without read access

Data bug:
- Promote actorName from @Transient to persisted VARCHAR column (V18 migration)
- Set actorName in notifyReply and notifyMentions from comment.getAuthorName()

Architecture:
- Add @RequirePermission(READ_ALL) to NotificationController
- Introduce NotificationDTO — controller returns DTO instead of Notification entity,
  eliminating lazy-load N+1 and AppUser field leakage
- Change mentions FetchType to EAGER — fixes LazyInitializationException outside transaction
- Add @Transactional(propagation=REQUIRES_NEW) to notifyReply/notifyMentions so a
  notification failure cannot roll back the parent comment
- N+1 fix: replace per-ID findById loops with single findAllById bulk fetch
- Move collectParticipantIds to CommentService; notifyReply accepts Set<UUID> directly

Security:
- Escape displayName before injecting into renderBody HTML span
- Replace <a href="#"> with <span class="mention"> — no profile page to link to, and
  the anchor's scroll-to-top behaviour is harmful

Tests added/fixed:
- markRead_throwsNotFound, markAllRead_delegatesToRepository, countUnread_delegatesToRepository
- markOneRead_returns401, @RequirePermission 403 coverage for both controllers
- postComment/replyToComment_triggersNotifyMentions_whenMentionedUserIdsProvided
- search_returnsAtMostTenResults now asserts $.length() <= 10
- XSS regression test for escaped displayName in mention.spec.ts

Frontend minors:
- relativeTime() uses Intl.RelativeTimeFormat (locale-aware, not German-hardcoded)
- aria-label uses m.notification_unread() Paraglide key (de/en/es added)
- <div role="button"> replaced with <button> (native Enter+Space handling)
- onDestroy clears debounceTimer in MentionEditor
- setTimeout(100) replaced with await tick() + requestAnimationFrame in CommentThread
- Notification prefs form uses checkbox name attributes + formData.has() pattern

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/CommentThread.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-import { onMount, untrack } from 'svelte';
+import { onMount, tick, untrack } from 'svelte';
 import { m } from '$lib/paraglide/messages.js';
 import type { Comment, CommentReply } from '$lib/types';
 import MentionEditor from '$lib/components/MentionEditor.svelte';
@@ -180,7 +180,7 @@ function cancelReply() {
 	replyText = '';
 }
 
-onMount(() => {
+onMount(async () => {
 	if (loadOnMount) {
 		reload();
 	} else {
@@ -189,11 +189,11 @@ onMount(() => {
 	}
 
 	if (targetCommentId) {
-		// Scroll to target after a tick so the DOM is settled
-		setTimeout(() => {
+		await tick();
+		requestAnimationFrame(() => {
 			const el = document.querySelector(`[data-comment-id="${targetCommentId}"]`);
 			el?.scrollIntoView({ behavior: 'smooth', block: 'center' });
-		}, 100);
+		});
 
 		// Remove highlight on first user interaction
 		const clearHighlight = () => {