Element keyboard shortcuts

Adds keyboard shortcuts, in the style of the WordPress block editor, for the element selected in the Navigator: duplicate (Cmd/Ctrl+Shift+D), cut (Cmd/Ctrl+X), add an element before or after it (Cmd/Ctrl+Opt/Alt+T / Cmd/Ctrl+Opt/Alt+Y, via a quick element picker), rename (F2), and edit text in the canvas (Enter to start, Escape to finish). A persistent canvas indicator shows when text editing is active. For elements without editable text, Enter lets you interact with the page and Escape returns to selecting elements. The native canvas-mode control also becomes keyboard operable. The new element actions appear in the right-click menu, alongside shortcuts to jump between the builder’s regions. Experimental: Builderius is adding its own shortcuts, so this may overlap or be retired.