Live activity indicator replacing typing dots (#96) (#104)

## Summary
- Closes [#96](#96).
- Parses versioned WS `status` frames in `MessageContext` (`activityStage` / `label` / `detail` / `history`).
- New `ActivityIndicator`: completed stages with checks, ~400ms min display, elapsed/still-working notes, a11y live region, reduced-motion, dots fallback when no frames.
- Wired through `ConversationDetailCard` + `AsyncDashboard2`; analytics `ACTIVITY_STAGE_COMPLETED`.

## Depends on
Backend status emission from [chat_backend PR](https://git.aimloperations.com/ai_ml_operations/chat_backend/pulls) (`feat/62-p4-evals-63-agentic`). Without it, UI keeps three-dot fallback.

## Test plan
- [x] Jest: `wsFrames`, `ActivityIndicator`, `ConversationDetailCard`, `MessageContext`
- [ ] Manual light/dark + 320px width
- [ ] Manual with backend status frames: Evaluating → Searching → Refining
- [ ] Manual: stream interrupt clears indicator
- [ ] Capacitor Android smokeReviewed-on: #104

llm-fe/ANALYTICS.md

@@ -31,6 +31,7 @@ Page views: always via `Tracker` / `tracker.js` on prod + beta.
 | 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 }` |
+| Activity Stage Completed | `ACTIVITY_STAGE_COMPLETED` | MessageContext | `{ stage, durationMs }` (no label/detail text) |
 
 ## Identify