Implement auto-lock behavior for mouse layer POC

[frangonf]

• Created new thumb_toggle behavior that uses toggle (&tog) instead of momentary (&mo) for layer activation
• Updated all base layer configurations to use thumb_toggle for mouse layer access
• Mouse layer now automatically locks when activated via thumb key and unlocks when pressed again
• Behavior mimics pressing "lock layer + mouse layer" but without needing a separate lock key press
• Applied to both macOS and Linux layer configurations across all keyboard layouts (QWERTY, Enthium, Engrammer, Engram, Dvorak, Colemak, ColemakDH)

Technical changes:

• Added thumb_toggle behavior in home-row-mods.dtsi (binds to &tog instead of &mo)
• Updated base-layers.dtsi: changed &thumb LAYER_Mouse to &thumb_toggle LAYER_Mouse
• Updated base-layers-linux.dtsi: changed &thumb LAYER_Mouse_Linux to &thumb_toggle LAYER_Mouse_Linux

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.