Fix: Ensure RetroArch saves are flushed before exiting via GameSwitcher

[Josh5]

Replaces SIGKILL with a graceful SIGTERM followed by a 5-second wait before force-killing RetroArch if necessary. This change allows RetroArch to complete its content close process and write save files to disk, resolving issues where saves were lost when quitting with GameSwitcher.

Fixes #1815