Fix proxy path resolution for portable builds, simplify landing page

The proxy launch used __dirname to find aware-cam-proxy.exe, which
points to a temp extraction directory in portable builds. Added
getAppDirectory() helper that resolves the actual .exe location via
PORTABLE_EXECUTABLE_FILE. Rewrote landing page as a simple kit download
page. Added build-kit script to bundle app + proxy + extension into zip.

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

package.json

@@ -8,6 +8,7 @@
     "dev": "electron . --dev",
     "build": "electron-builder --win --publish=never",
     "build-test": "electron-builder --win --dir",
+    "build-kit": "powershell -ExecutionPolicy Bypass -File build-kit.ps1",
     "prebuild": "echo Checking build requirements..."
   },
   "build": {