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/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 } from './mentions';
+import { MentionsView, MentionToastsView } from './mentions';
 
 export const MainView: FC<{}> = props =>
 {
@@ -242,6 +242,8 @@ 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 />
         </>
     );