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/components/MainView.tsx

@@ -48,7 +48,7 @@ import { UserAccountSettingsView } from './user-settings/UserAccountSettingsView
 import { UserSettingsView } from './user-settings/UserSettingsView';
 import { WiredView } from './wired/WiredView';
 import { WiredCreatorToolsView } from './wired-tools/WiredCreatorToolsView';
-import { MentionsView, MentionToastsView } from './mentions';
+import { MentionsView } from './mentions';
 
 export const MainView: FC<{}> = props =>
 {
@@ -242,8 +242,6 @@ export const MainView: FC<{}> = props =>
             { GetConfigurationValue<boolean>('radio_ui.enabled', false) && <RadioView /> }
             { (GetConfigurationValue<boolean>('mentions_ui.enabled', true) && mentionsVisible) &&
                 <MentionsView onClose={ () => setMentionsVisible(false) } /> }
-            { GetConfigurationValue<boolean>('mentions_ui.enabled', true) &&
-                <MentionToastsView /> }
             <ExternalPluginLoader />
         </>
     );