feat(displays): add custom keyboard overlay placement

[esphynox]

Summary

Adds a custom keyboard overlay placement option for displays. Users can choose Custom from the display anchor picker, open an arrange window with a live keyboard visualizer preview, drag it to the desired display/position, and apply the resulting placement.

The implementation stores custom placement as normalized display-frame proportions, syncs the selected display when the preview is moved across screens, applies placement when dragging stops, and finishes positioning when leaving the Displays pane. It also localizes the new UI, removes the old keyboard position picker, moves keyboard size into General, and adds shared geometry helpers used by the placement flow.

Closes #57.

Tests

• tuist generate
• xcodebuild test -project Keyty.xcodeproj -scheme Keyty -destination 'platform=macOS'
• Other:

Run project commands from Apps/Keyty.

UI Changes

Visible UI changes:

• Displays anchor picker now includes Custom.
• Custom placement shows a Position row with an Arrange... / Apply flow.
• The arrange window previews the keyboard visualizer with T, E, S, T keycaps.
• Keyboard Size moved into the Keyboard General section; the empty Layout section was removed.

Documentation

• Updated relevant docs
• No docs needed

Release Notes

Adds custom placement for the keyboard overlay so users can arrange the visualizer at a precise position on a chosen display.