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/room/widgets/chat-input/ChatInputMentionSelectorView.tsx

@@ -1,17 +1,6 @@
 import { FC, useEffect, useRef } from 'react';
 import { LayoutAvatarImageView } from '../../../../common';
-
-export type MentionSuggestionKind = 'user' | 'alias';
-
-export interface MentionSuggestion
-{
-    key: string;
-    kind: MentionSuggestionKind;
-    name: string;
-    insertToken: string;
-    figure?: string;
-    description?: string;
-}
+import { MentionSuggestion } from '../../../../hooks/rooms/widgets/useChatMentions.helpers';
 
 interface ChatInputMentionSelectorViewProps
 {