Add performance helper FPS cap

[Babyhamsta]

Summary

• Add an AI FPS limit slider under Model Settings with unlimited default behavior.
• Add the Performance Helper prompt/window, benchmark runner, resource sampling, and goal-based recommendations.
• Harden model reload, benchmark cleanup, cancellation, and helper state persistence.

Validation

• dotnet build Aimmy2.sln -c Release -p:Platform=x64 -v:minimal
• git diff --cached --check
• Agent-assisted bug scan plus local confirmation of fixes