fix: code splitting, route errors, Sentry CI source maps

- Lazy-import CreateRoomForm/CreateSpaceForm in CreateRoom.tsx and Create.tsx
  so create-room and create-space get their own chunks; eliminates
  INEFFECTIVE_DYNAMIC_IMPORT warnings
- Add RouteError component wired to root route errorElement so crashes show
  a reload button instead of React Router dev screen
- ci.yml: use secrets.SENTRY_AUTH_TOKEN so source maps upload on CI builds

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

src/app/pages/RouteError.tsx

@@ -0,0 +1,43 @@
+import React from 'react';
+import { useRouteError, isRouteErrorResponse } from 'react-router-dom';
+
+export function RouteError() {
+  const error = useRouteError();
+
+  const message = isRouteErrorResponse(error)
+    ? `${error.status} ${error.statusText}`
+    : error instanceof Error
+      ? error.message
+      : 'An unexpected error occurred.';
+
+  return (
+    <div
+      style={{
+        display: 'flex',
+        flexDirection: 'column',
+        alignItems: 'center',
+        justifyContent: 'center',
+        height: '100dvh',
+        gap: '16px',
+        padding: '32px',
+        fontFamily: 'sans-serif',
+      }}
+    >
+      <h2 style={{ margin: 0, fontSize: '1.25rem' }}>Something went wrong</h2>
+      <p style={{ margin: 0, opacity: 0.7, textAlign: 'center' }}>{message}</p>
+      <button
+        type="button"
+        onClick={() => window.location.reload()}
+        style={{
+          padding: '8px 20px',
+          borderRadius: '8px',
+          border: 'none',
+          cursor: 'pointer',
+          fontWeight: 600,
+        }}
+      >
+        Reload
+      </button>
+    </div>
+  );
+}