feat(#71): add notification bell + preferences UI

- NotificationBell.svelte: bell icon in header with unread badge, dropdown showing last 10 notifications, mark-all-read, click-outside close, keyboard Escape support, polls every PUBLIC_NOTIFICATION_POLL_MS ms
- Wire NotificationBell into +layout.svelte between ThemeToggle and UserMenu (authenticated users only)
- Profile page: add notification preferences card with notifyOnReply / notifyOnMention toggles, loaded via GET and saved via PUT /api/users/me/notification-preferences
- i18n: de/en/es message keys for bell, notifications list, and preference labels

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

frontend/messages/en.json

@@ -294,5 +294,16 @@
 	"enrich_done_body": "All documents have been processed.",
 	"enrich_back_to_list": "Back to list",
 	"comment_empty_hint": "No comments yet – start the discussion!",
-	"comment_start_discussion": "Start discussion →"
+	"comment_start_discussion": "Start discussion →",
+	"notification_bell_label": "Notifications",
+	"notification_bell_unread_label": "{count} unread notifications",
+	"notification_mark_all_read": "Mark all read",
+	"notification_empty": "No new notifications",
+	"notification_type_reply": "{actor} replied to your comment",
+	"notification_type_mention": "{actor} mentioned you in a comment",
+	"notification_prefs_heading": "Notifications",
+	"notification_pref_reply": "Email when someone replies to my comment",
+	"notification_pref_mention": "Email when someone mentions me in a comment",
+	"mention_btn_label": "Mention person",
+	"mention_popup_empty": "No users found"
 }