Fix start with system logon launch

[enaboapps]

Summary

• replace HKCU Run as the primary Start with system registration with a per-user logon scheduled task
• migrate healthy legacy HKCU Run registrations to the scheduled task and clean up old Run/StartupApproved entries
• expand startup diagnostics and Settings copy for scheduled task state
• add focused scheduled task, startup service, settings, and diagnostics tests

Closes #307.

Validation

• dotnet restore src/SwitchifyPc.sln
• dotnet build src/SwitchifyPc.sln -c Release --no-restore
• dotnet test src/SwitchifyPc.sln -c Release --no-build