Make speculative decoding default to Auto with explicit disable support

[thomas9120]

Summary

• Change Speculative Type default from None to Auto.
• Add explicit None behavior that emits --spec-type none and suppresses draft/speculative arguments.
• Preserve saved draft and n-gram settings when switching between Auto and None.
• Normalize legacy n-gram-only presets to Auto with independent toggles.
• Update documentation and frontend coverage.

Testing

• Added launch-argument unit coverage for Auto, None, and restored speculative settings.
• Expanded frontend smoke coverage for Configure state synchronization and command preview behavior.