feat: migrate embedded frontend from custom MVX framework to React 18

Replace the custom MVX rendering framework (BaseComponent, StateStore,
Compositor, MVX orchestrator) with React 18 functional components using
an adapter pattern that preserves the existing public API.

## What changed

- **Adapter pattern**: ReactCaptureMenu and ReactResultView implement the
  exact same public API as the original classes, using ExternalStore +
  useSyncExternalStore for state management instead of MVX state IDs
- **32 React components** replace 31 MVX components across CaptureMenu
  (5 components) and ResultView (18 components + JSONRenderTree)
- **Visual regression tests**: 38 Playwright screenshot tests ensure
  pixel-level visual parity with the original UI
- **SCSS unchanged**: Same 3 SCSS files, same class names, same DOM
  structure — zero CSS changes needed
- **Ace editor**: Wrapped with useRef/useEffect for proper lifecycle
  management (no StrictMode — incompatible with Ace)

## Architecture

- ExternalStore<T> bridges imperative API → React reactivity
- Adapter classes translate method calls → store.setState() → re-render
- React components are purely presentational (read store, call handlers)
- Context pattern passes adapter instance to component tree
- JSONRenderItem discriminated union for recursive JSON tree rendering

## Bug fixes

- hide() now actually works (MVX had truthy-check bug)
- Removed dead IE11 navigator.msSaveBlob code path
- Fixed falsy number rendering in VisualStateListItem (textureLayer=0)

## Bundle impact

- Before: 667 KB (MVX + Ace)
- After: 794 KB (React + ReactDOM + Ace) — +127 KB for React

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sebavan

.gitignore

@@ -8,4 +8,4 @@
 /node_modules
 npm-debug.log.**
 npm-debug.log
-.DS_Store
+.DS_Storeplaywright-report/

dist/spector.bundle.js

@@ -0,0 +1,39 @@
+/**
+ * @license React
+ * react-dom.production.min.js
+ *
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+/**
+ * @license React
+ * react-jsx-runtime.production.min.js
+ *
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+/**
+ * @license React
+ * react.production.min.js
+ *
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+/**
+ * @license React
+ * scheduler.production.min.js
+ *
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */