Add Flip Horizontal and Flip Vertical UI

[pascalpp]

Summary

Add the user-facing commands for changing the ruler zero corner.

Implementation Notes

• Add menu actions named Flip Horizontal and Flip Vertical.
• Add focused-ruler hotkeys: Shift+H and Shift+V.
• Add hotkey bezel feedback for the resulting state.
• Keep UI language action-oriented; do not expose internal “zero corner” terminology unless it is clearly helpful.
• Localize menu, bezel, and help strings.
• Update the Help shortcut page.

Acceptance Criteria

• Menu actions are available and wired to the correct AppDelegate actions.
• Shift+H and Shift+V work while a ruler window is focused.
• Hotkey bezel feedback appears.
• Help and localization resources are updated consistently.

[pascalpp]

Close #205