Activate focused buttons with Enter

[shytzedaka]

What changed

• Handle both Return and numeric-keypad Enter in the shared InstallerButton component.
• Activate the focused enabled button once per key press.
• Preserve Qt's existing Space-key behavior and keep disabled buttons inert.

Root cause

Qt's button handling activated these custom installer buttons with Space, but Enter was not mapped to the button's clicked action.

Impact

Every installer action using the shared button component can now be confirmed with Space, Return, or keypad Enter.

Validation

• git diff --check
• Verified the handler accepts only Return/Enter, rejects auto-repeat, and checks the enabled state.