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/mentionsFormat.ts

@@ -0,0 +1,41 @@
+// Date/time helpers for the mentions box. Kept framework-free and pure so they
+// are unit-testable. Timestamps are unix SECONDS (as carried on the wire).
+
+export type MentionDateGroup = 'today' | 'yesterday' | 'older';
+
+const DAY_MS = 86_400_000;
+
+const startOfDay = (d: Date): number => new Date(d.getFullYear(), d.getMonth(), d.getDate()).getTime();
+
+const pad = (n: number): string => (n < 10 ? `0${ n }` : `${ n }`);
+
+/**
+ * Bucket a mention timestamp into today / yesterday / older relative to `now`.
+ */
+export const getMentionDateGroup = (timestampSeconds: number, now: Date = new Date()): MentionDateGroup =>
+{
+    if(!timestampSeconds || timestampSeconds <= 0) return 'older';
+
+    const ts = timestampSeconds * 1000;
+    const todayStart = startOfDay(now);
+
+    if(ts >= todayStart) return 'today';
+    if(ts >= (todayStart - DAY_MS)) return 'yesterday';
+
+    return 'older';
+};
+
+/**
+ * Compact per-row time label: HH:MM for today/yesterday (the section header
+ * disambiguates the day), DD-MM for older entries. Empty string when unknown.
+ */
+export const formatMentionTime = (timestampSeconds: number, now: Date = new Date()): string =>
+{
+    if(!timestampSeconds || timestampSeconds <= 0) return '';
+
+    const d = new Date(timestampSeconds * 1000);
+
+    if(getMentionDateGroup(timestampSeconds, now) === 'older') return `${ pad(d.getDate()) }-${ pad(d.getMonth() + 1) }`;
+
+    return `${ pad(d.getHours()) }:${ pad(d.getMinutes()) }`;
+};