feat: add Sentry error tracking with defensive error boundary

- Initialize Sentry SDK in index.tsx when VITE_SENTRY_DSN env var is set
- Wrap entire App with Sentry.ErrorBoundary (replaces the hard crash with a retry UI)
- 5% trace sample rate, sendDefaultPii disabled, strip events containing accessToken
- Add .env.production template with VITE_SENTRY_DSN placeholder
- Get your DSN from sentry.io -> Project Settings -> Client Keys

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "lotus-chat",
   "version": "4.12.1-lotus",
-  "description": "Lotus Chat \u2014 Matrix client for Lotus Guild",
+  "description": "Lotus Chat — Matrix client for Lotus Guild",
   "main": "index.js",
   "type": "module",
   "engines": {
@@ -70,6 +70,7 @@
     "@giphy/js-fetch-api": "5.8.0",
     "@giphy/js-types": "5.1.0",
     "@giphy/react-components": "10.1.2",
+    "@sentry/react": "10.53.1",
     "@tanstack/react-query": "5.24.1",
     "@tanstack/react-query-devtools": "5.24.1",
     "@tanstack/react-virtual": "3.2.0",