Add brand-specific update channel and environment configuration

- Updated `brand_config.py` to include `WEBDROP_UPDATE_CHANNEL` in the environment variables.
- Enhanced `build_macos.sh` to create a bundled `.env` file with brand-specific defaults, including the update channel.
- Implemented a method in `build_windows.py` to create a bundled `.env` file for Windows builds, incorporating brand-specific runtime defaults.
- Modified `config.py` to ensure the application can locate the `.env` file in various installation scenarios.
- Added unit tests in `test_config.py` to verify the loading of the bootstrap `.env` from the PyInstaller runtime directory.
- Generated new WiX object and script files for the Windows installer, including application shortcuts and registry entries.

build/scripts/brand_config.py

@@ -269,6 +269,7 @@ def cli_env(args: argparse.Namespace) -> int:
         "WEBDROP_EXE_NAME": brand.exe_name,
         "WEBDROP_BUNDLE_ID": brand.bundle_identifier,
         "WEBDROP_CONFIG_DIR_NAME": brand.config_dir_name,
+        "WEBDROP_UPDATE_CHANNEL": brand.update_channel,
         "WEBDROP_ICON_ICO": str(brand.icon_ico),
         "WEBDROP_ICON_ICNS": str(brand.icon_icns),
     }