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/api/notification/MentionNotificationBubbleItem.ts

@@ -0,0 +1,25 @@
+import { IMentionEntry } from '../mentions';
+import { NotificationBubbleItem } from './NotificationBubbleItem';
+import { NotificationBubbleType } from './NotificationBubbleType';
+
+/**
+ * A notification bubble that carries a full mention entry, so the dedicated
+ * mention bubble layout can render the sender's avatar (from the figure) and
+ * the go-to-room action — data the plain NotificationBubbleItem can't hold.
+ */
+export class MentionNotificationBubbleItem extends NotificationBubbleItem
+{
+    private _mention: IMentionEntry;
+
+    constructor(mention: IMentionEntry)
+    {
+        super(mention.message, NotificationBubbleType.MENTION, null, null, mention.senderUsername);
+
+        this._mention = mention;
+    }
+
+    public get mention(): IMentionEntry
+    {
+        return this._mention;
+    }
+}