feat(mentions): overhaul, refactor, notification bubble & window update

Chat tagging:
- Any @user is a visible tag in chat bubbles (the .mention-tag CSS never
  existed, so highlighting was invisible); self/alias mentions get a gold
  emphasis. Fixes cross-room tags not being highlighted.

Mentions window:
- Redesigned: unread count in the header, restyled filter chips + a refresh
  button, CSS-driven list/date-groups, adaptive height (compact when few,
  capped + scroll when many), polished empty state.
- Rows: framed avatar (friends-list head crop so the face is never clipped),
  per-row unread dot, type marker, icon action buttons (goto / remove).
- Re-requests from the server each time it opens.

Autocomplete:
- Never suggests the viewer themselves; suggests room users + online friends +
  aliases.

Notifications:
- Mention toast removed; mentions flow through the client's standard
  notification stream via a dedicated mention bubble (avatar + actions) in the
  default position. EVERY received mention surfaces (independent of the generic
  info-feed toggle, gated only by mentions_ui.enabled).

Refactor (behaviour-preserving):
- Centralised @-token classification in api/mentions/mentionTokens.
- Moved mentionsFormat -> api/mentions, useMentionActions -> hooks/mentions.
- Extracted ChatInputView @-autocomplete into a tested useChatMentions hook +
  pure helper; removed the dead duplicate useMentionAutocomplete.

src/components/notification-center/views/bubble-layouts/GetBubbleLayout.tsx

@@ -1,7 +1,8 @@
-import { NotificationBubbleItem, NotificationBubbleType } from '../../../../api';
+import { MentionNotificationBubbleItem, NotificationBubbleItem, NotificationBubbleType } from '../../../../api';
 import { NotificationBadgeReceivedBubbleView } from './NotificationBadgeReceivedBubbleView';
 import { NotificationClubGiftBubbleView } from './NotificationClubGiftBubbleView';
 import { NotificationDefaultBubbleView } from './NotificationDefaultBubbleView';
+import { NotificationMentionBubbleView } from './NotificationMentionBubbleView';
 
 export const GetBubbleLayout = (item: NotificationBubbleItem, onClose: () => void) =>
 {
@@ -15,6 +16,8 @@ export const GetBubbleLayout = (item: NotificationBubbleItem, onClose: () => voi
             return <NotificationBadgeReceivedBubbleView key={ item.id } { ...props } />;
         case NotificationBubbleType.CLUBGIFT:
             return <NotificationClubGiftBubbleView key={ item.id } { ...props } />;
+        case NotificationBubbleType.MENTION:
+            return <NotificationMentionBubbleView key={ item.id } item={ item as MentionNotificationBubbleItem } onClose={ onClose } />;
         default:
             return <NotificationDefaultBubbleView key={ item.id } { ...props } />;
     }