feat(mentions): @ autocomplete, blue @nick, avatar notification toast

- Chat input @ autocomplete: typing @ shows online users (room users +
  online friends + room aliases) with avatars; arrows/Tab/Enter to pick.
- Any valid @nick token is highlighted blue in chat bubbles (like @all),
  giving visual feedback that it is a recognised mention.
- Side notification toast on a received mention: sender avatar (from the
  new senderFigure wire field) + message + dismiss; dismiss marks it read
  so the toolbar unread badge updates. Auto-hides after 8s.
- IMentionEntry/parsers carry senderFigure end to end.

src/hooks/mentions/mentionToastsStore.ts

@@ -0,0 +1,58 @@
+import { IMentionEntry } from '../../api';
+
+// Toast laterali per le menzioni appena ricevute (avatar + messaggio + dismiss).
+// Separato da mentionsStore: i toast sono effimeri, le menzioni persistono nel pannello.
+export interface MentionToast
+{
+    mentionId: number;
+    senderId: number;
+    senderUsername: string;
+    senderFigure: string;
+    message: string;
+    roomName: string;
+}
+
+const MAX_TOASTS = 4;
+
+let toasts: MentionToast[] = [];
+const listeners = new Set<() => void>();
+
+const emit = (): void =>
+{
+    for(const listener of listeners) listener();
+};
+
+export const subscribeMentionToasts = (callback: () => void): (() => void) =>
+{
+    listeners.add(callback);
+    return () => { listeners.delete(callback); };
+};
+
+export const getMentionToasts = (): ReadonlyArray<MentionToast> => toasts;
+
+export const pushMentionToast = (entry: IMentionEntry): void =>
+{
+    toasts = [
+        {
+            mentionId: entry.mentionId,
+            senderId: entry.senderId,
+            senderUsername: entry.senderUsername,
+            senderFigure: entry.senderFigure,
+            message: entry.message,
+            roomName: entry.roomName
+        },
+        ...toasts.filter(toast => toast.mentionId !== entry.mentionId)
+    ].slice(0, MAX_TOASTS);
+
+    emit();
+};
+
+export const dismissMentionToast = (mentionId: number): void =>
+{
+    const next = toasts.filter(toast => toast.mentionId !== mentionId);
+
+    if(next.length === toasts.length) return;
+
+    toasts = next;
+    emit();
+};