Add global option to disable automatic controller switching

[adamrb]

Summary

• Adds an "Auto-Switch Controllers" toggle in Global Settings (default: enabled) that controls whether the active controller automatically switches when input is detected from a different controller
• When disabled, a "Current Controller" dropdown appears allowing manual selection from connected controllers (including a "Keyboard & Mouse" option)
• The dropdown dynamically refreshes when controllers are hotplugged/unplugged while the settings screen is open
• Bumps version to 3.0.0-beta.3

Motivation

The automatic controller switching makes it impossible to run multiple game instances side-by-side with different controllers, since pressing a button on controller B in one instance causes the other instance to also switch to controller B.

Test plan

• Open Global Settings and verify "Auto-Switch Controllers" toggle appears in Miscellaneous, defaulting to enabled
• With toggle enabled: press a button on controller B while controller A is active — B should become active
• With toggle disabled: press a button on controller B while controller A is active — A should remain active
• With toggle disabled: verify "Current Controller" dropdown appears and lists all connected controllers
• Hotplug a controller while settings screen is open — dropdown should update
• Verify setting persists after restart (check config JSON for auto_switch_controllers)