Add Electron-based test application for CesiumJS MCP servers

[MKartaviciute]

Summary

Add an Electron test application alongside the existing \web-app under \test-applications\. This allows testing and demonstrating the CesiumJS MCP servers in a desktop app context without requiring a browser or an \http-server\ process.

Motivation

The current web-app must be served via http-server (port 8080) and opened in a browser. An Electron app:

• Removes the need for a separate serve step the app launches as a single desktop process
• Provides a more realistic IDE-integration story (e.g. launching alongside Claude Desktop or Cursor)
• Enables future work such as packaging and distributing the test app as a standalone executable
• Makes it easier to demo all MCP servers (camera, entity, animation, imagery, tiles, terrain) without browser security restrictions on localhost WebSocket connections

Proposed Implementation

New package: /test-applications/electron-app/

test-applications/
  electron-app/
    package.json               # electron, electron-builder, esbuild deps
    tsconfig.json
    electron/
      main.ts                  # BrowserWindow setup, app lifecycle
      preload.ts               # contextBridge (if IPC needed)
    src/
      index.html               # renderer entry (bundles CesiumJS locally)
      app.ts                   # thin wrapper — reuses @cesium-mcp/client-core
    assets/
      cesium/                  # copied CesiumJS build assets (WASM, workers, etc.)

Workspace integration

• Add electron-app to the test-applications pnpm workspace
• Add workspace-level scripts in test-applications\package.json for start:electron and build:electron
• Add .env support

Out of Scope

• Packaging / distributing a production-ready installer (can be a follow-up)
• Electron IPC between main and renderer (not needed; MCP servers communicate over TCP ports)
• Changing any existing MCP server code

Acceptance Criteria

• pnpm run start:electron (from test-applications) launches the Electron window with a working CesiumJS globe
• All 6 MCP servers connect successfully via WebSocket (green status panel)
• No changes to @cesium-mcp/client-core or any server package
• pnpm run check passes (lint + format)
• README updated with Electron setup/run instructions