feat(installer): add install detection and uninstaller launch support

[kev1n77]

• Detect existing installations from registry and surface version/path in the options step
• Add a button to launch the registered uninstaller
• Use bitfun-desktop.exe consistently across payload, shortcuts, launch, and registry logic
• Write and clean up install location registry entries and uninstall keys
• Remove PATH and context menu install options while keeping legacy cleanup during uninstall
• Add installer default context window when saving first-launch model config