Add auto software fallback when the SimDeck client loses foreground

[DjDeveloperr]

Summary

• Track the active stream client foreground state and switch the encoder session to software whenever the SimDeck web client is not foreground.
• Keep the user-selected encoder mode intact while only overriding the active session behavior during background periods.
• Expose the foreground-driven encoder state in native encoder stats so health and metrics reflect the active mode.
• Update video and troubleshooting docs to describe the new auto behavior.

Testing

• Not run (not requested)
• Verified the affected native and Rust code paths are wired through the existing session and encoder control flow.