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/mentions/mentionTokens.ts

@@ -0,0 +1,50 @@
+// Shared @-mention token classification, used by both the chat-bubble
+// highlighter and the mentions panel so the two can't diverge.
+
+export const MENTION_ROOM_ALIASES: ReadonlyArray<string> = [
+    'all', 'everyone', 'tutti',
+    'friends', 'amici',
+    'room', 'stanza'
+];
+
+const NON_NICK_CHARS = /[^A-Za-z0-9_]/g;
+
+const normalizeToken = (token: string): string =>
+{
+    if(!token || (token.length < 2) || (token.charAt(0) !== '@')) return '';
+
+    return token.substring(1).replace(NON_NICK_CHARS, '').toLowerCase();
+};
+
+const normalizeNick = (value: string): string => (value || '').replace(NON_NICK_CHARS, '').toLowerCase();
+
+// '' = not a mention; 'tag' = any @user (subtle chip); 'self' = the token
+// targets the viewer (own nick) or is a broadcast alias (strong highlight).
+export type MentionKind = '' | 'tag' | 'self';
+
+export const classifyMentionToken = (
+    token: string,
+    ownUsername: string,
+    aliases: ReadonlyArray<string> = MENTION_ROOM_ALIASES
+): MentionKind =>
+{
+    const nick = normalizeToken(token);
+
+    if(!nick) return '';
+
+    const ownLower = normalizeNick(ownUsername);
+
+    if((ownLower && (nick === ownLower)) || aliases.some(alias => alias.toLowerCase() === nick)) return 'self';
+
+    return 'tag';
+};
+
+/**
+ * Back-compat boolean — true only when the token targets the viewer or a
+ * broadcast alias (i.e. "I was mentioned"), not for generic @user tags.
+ */
+export const tokenIsMention = (
+    token: string,
+    ownUsername: string,
+    aliases: ReadonlyArray<string> = MENTION_ROOM_ALIASES
+): boolean => (classifyMentionToken(token, ownUsername, aliases) === 'self');