Add option to disable keyboard shortcuts in viewer

[Shivam-Bhardwaj]

Summary

• Add handle_keyboard parameter to launch() and launch_passive() functions
• When set to False, built-in keyboard shortcuts (e.g., 'W' for wireframe, space for pause) are disabled
• Useful for robot teleoperation where keyboard inputs should be handled by user code

Closes #2953

Changes

• simulate/simulate.h: Add keyboard_ui flag
• simulate/simulate.cc: Check flag before processing keyboard events
• python/mujoco/simulate.cc: Expose keyboard_ui property in Python bindings
• python/mujoco/viewer.py: Add handle_keyboard parameter to launch functions

Example usage

# Disable keyboard shortcuts for teleoperation
viewer = mujoco.viewer.launch_passive(
    model, data,
    handle_keyboard=False,  # Disable built-in shortcuts
    key_callback=my_teleop_callback  # Handle keys in user code
)

Test plan

• Code follows existing patterns for ui0_enable/ui1_enable
• Ran pyink and isort for style compliance
• Manual testing with passive viewer (requires display)