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/hooks/mentions/useMentionActions.ts

@@ -0,0 +1,39 @@
+import { CreateLinkEvent, DeleteMentionComposer, MarkMentionsReadComposer } from '@nitrots/nitro-renderer';
+import { useMemo } from 'react';
+import { IMentionEntry, SendMessageComposer } from '../../api';
+import { markRead, removeMention } from './mentionsStore';
+
+export interface MentionActions
+{
+    /** Row click: mark the mention as read (no navigation). */
+    open: (mention: IMentionEntry) => void;
+    /** Explicit "go to room" action: mark read, then jump to the origin room. */
+    goto: (mention: IMentionEntry) => void;
+    /** Permanently delete the mention server-side (DeleteMentionComposer) and
+     *  drop it from the local list, so it does not reappear after a relog. */
+    remove: (mention: IMentionEntry) => void;
+}
+
+const markReadOnServer = (mention: IMentionEntry): void =>
+{
+    if(mention.read) return;
+    markRead(mention.mentionId);
+    SendMessageComposer(new MarkMentionsReadComposer(1, mention.mentionId));
+};
+
+// Shared action handlers used by both MentionsView and the chat-history
+// "Menzioni" tab so behaviour can't diverge.
+export const useMentionActions = (): MentionActions => useMemo(() => ({
+    open: (mention) => markReadOnServer(mention),
+    goto: (mention) =>
+    {
+        markReadOnServer(mention);
+        if(mention.roomId > 0) CreateLinkEvent(`navigator/goto/${ mention.roomId }`);
+    },
+    remove: (mention) =>
+    {
+        // Permanent server-side delete, then drop it from the local list.
+        SendMessageComposer(new DeleteMentionComposer(mention.mentionId));
+        removeMention(mention.mentionId);
+    }
+}), []);