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/ANALYTICS.md

@@ -27,6 +27,10 @@ Page views: always via `Tracker` / `tracker.js` on prod + beta.
 | Account Delete Started | `ACCOUNT_DELETE_STARTED` | DeleteAccountSection | — |
 | Account Delete Success | `ACCOUNT_DELETE_SUCCESS` | DeleteAccountSection | — |
 | Account Delete Failed | `ACCOUNT_DELETE_FAILED` | DeleteAccountSection | — |
+| Message Copied | `MESSAGE_COPIED` | MessageActions | `{ role }` |
+| Message Rated | `MESSAGE_RATED` | MessageActions | `{ rating: 'up' \| 'down' \| 'cleared' }` |
+| Message Rating Reason | `MESSAGE_RATING_REASON` | MessageActions | `{ reason, hasComment }` |
+| Message Exported | `MESSAGE_EXPORTED` | MessageActions, AsyncDashboard2 | `{ format, scope }` |
 
 ## Identify