Major rewrite from the 2022 .NET 6 / NSIS-packaged release:

- Ported to .NET 8 LTS.
- Async-all-the-way HTTP (was sync-over-async, deadlock-prone).
- Config-driven via appsettings.json (was hardcoded poll interval, extension id, etc.).
- DRY'd toggle logic (three near-duplicate branches collapsed).
- Optional X-API-Key header to match the WebApi's new auth.
- Desktop-shortcut icon bug fixed (<ApplicationIcon> added to csproj).
- New WiX v6 MSI installer with custom config dialog, silent-install
  support, Add/Remove Programs integration, Major Upgrade semantics,
  autostart via all-users Startup folder, launch-on-finish.
- tools/cleanup-legacy-install.ps1 for migrating off the NSIS install.
- Rewritten README, new docs/installer-design.md.