feat: instant-launch SOCDOF installer and startup splash

[Strudelcode]

Summary

• Add a lightweight .NET Framework 4.8 Windows bootstrapper with a dark, Discord-inspired setup window that appears immediately without bundling another full .NET runtime.
• Fetch the latest official SOCDOF setup from GitHub Releases and show real download progress.
• Let users choose the installation directory; default to %LOCALAPPDATA%\\Programs\\SOCDOF.
• Run the packaged NSIS setup silently, show installation status, and offer to launch SOCDOF when complete.
• Keep the in-app startup splash so launching the installed desktop app also gives immediate visual feedback.
• Update the desktop download UI to prefer SOCDOF-Installer.exe and document the changes.

Verification

• Added a Windows validation workflow for TypeScript/build checks and the .NET Framework bootstrapper compile.
• CI results and hands-on Windows tests still need confirmation: fresh install, custom path, upgrade, offline/error handling, and first launch.

The bootstrapper needs internet access to fetch the latest setup. SOCDOF itself remains offline-capable after installation.