Invisible Item Frames

Initial public release of the gesture interaction model:

• Toggle item frames and signs invisible: keybind (default Left Alt) + right-click, or Shift + right-click. Empty main hand required.
• Independent click-through for visible/invisible frames and signs: plain right-click reaches the block behind instead.
• Swap option reverses the Shift and keybind roles.
• Config via ModMenu/Cloth Config screen or /invisibleitemframes config; opt-in permission gate for toggling.

Fixed before release: the keybind did not work on item frames, and Shift-clicking a sign with anything in the offhand (e.g. a shield) silently did nothing instead of opening the text editor.