feat(share-ai): label public chat with the assistant identity name

The anonymous public-share "Ask AI" chat labeled every assistant turn
with the generic "AI agent" even when an Assistant identity (agent role)
was configured. Surface the configured identity name instead, falling
back to "AI agent" when no identity is set.

- server: AiSettingsService.resolvePublicShareAssistantName resolves the
  configured role's name (null when unset/missing/disabled), mirroring
  PublicShareChatService.resolveShareRole; ShareController returns it as
  aiAssistantName on /shares/page-info (only when the assistant is on).
- client: thread aiAssistantName -> ShareAiWidget -> MessageList ->
  MessageItem/TypingIndicator via an optional assistantName prop; the
  internal chat omits it and keeps showing "AI agent".
- i18n: add "{{name}} is typing…" (en-US, ru-RU) for the typing line.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/src/features/share/components/share-ai-widget.tsx

@@ -27,6 +27,8 @@ interface ShareAiWidgetProps {
   shareId: string;
   /** The page the reader currently has open (context for "this page"). */
   pageId: string;
+  /** Display name of the configured assistant identity; falls back to 'AI agent' when absent. */
+  assistantName?: string;
 }
 
 /**
@@ -47,7 +49,11 @@ interface ShareAiWidgetProps {
  * links (so internal UUIDs/auth-gated routes in the answer don't leak as
  * clickable links), and a documentation-focused empty state.
  */
-export default function ShareAiWidget({ shareId, pageId }: ShareAiWidgetProps) {
+export default function ShareAiWidget({
+  shareId,
+  pageId,
+  assistantName,
+}: ShareAiWidgetProps) {
   const { t } = useTranslation();
   const [open, setOpen] = useState(false);
   const [input, setInput] = useState("");
@@ -152,6 +158,7 @@ export default function ShareAiWidget({ shareId, pageId }: ShareAiWidgetProps) {
           <MessageList
             messages={messages}
             isStreaming={isStreaming}
+            assistantName={assistantName}
             showCitations={false}
             // Anonymous reader: neutralize internal/relative links in the
             // assistant's markdown so internal UUIDs/auth-gated routes don't