Message actions + citation Sources (#97, #98) (#101)

## Summary
- Closes [#97](#97) — message action row: copy (raw markdown + code-block copy), thumbs up/down with optimistic updates + down-reason popover, and export menu (PDF / DOCX / CSV / XLSX / TXT) for a single message plus conversation-level export in the chat toolbar.
- Closes [#98](#98) — versioned WS `citations` frame parsing, Sources list under assistant bubbles, clickable `[n]` markers, and history hydrate from `Prompt.citations`.
- Backend companion for ratings: [chat_backend#67](ai_ml_operations/chat_backend#67). Thumbs UI posts to `prompt_feedback` once that lands; votes rehydrate from `conversation_details.feedback`.
- Export libs (`pdfmake`, `docx`, `papaparse`, `xlsx`) are dynamically imported so they stay out of the main path until used.

## Test plan
- [ ] Stream a grounded answer → Sources appear after stream; inline `[n]` highlights the matching source; reload keeps Sources.
- [ ] Non-grounded turn → no Sources header.
- [ ] Copy message + code block; confirm checkmark ~2s; failure path shows toast.
- [ ] Thumbs up/down optimistic UI; clear by re-click; down opens reason popover without blocking the vote (needs chat_backend#67).
- [ ] Export one message and whole conversation in all five formats; check searchable PDF text and DOCX structure.
- [ ] Action row: hover reveal on desktop, always visible on last/touch; hidden while streaming.
- [ ] `npm test -- --testPathPattern='wsFrames|clipboard|exportChat|promptFeedback|ConversationDetailCard'`Reviewed-on: #101

llm-fe/src/App.tsx

@@ -24,6 +24,7 @@ import { AnalyticsConsentProvider } from './llm-fe/contexts/AnalyticsConsentCont
 import AnalyticsConsentBanner from './llm-fe/components/AnalyticsConsentBanner/AnalyticsConsentBanner';
 import AnalyticsSession from './llm-fe/components/AnalyticsSession/AnalyticsSession';
 import AppErrorBoundary from './llm-fe/components/AppErrorBoundary/AppErrorBoundary';
+import ToastHost from './llm-fe/components/ToastHost/ToastHost';
 
 const ProtectedRoutes = () => {
   const { authenticated, loading } = useContext(AuthContext);
@@ -48,6 +49,7 @@ class App extends Component {
         <AnalyticsConsentProvider>
         <Tracker />
         <AnalyticsSession />
+        <ToastHost />
         <AnalyticsConsentBanner />
         <div className='site'>
           <main>