allozaur/feat/chat form contenteditable (#26717)

* feat: Add contenteditable tokenizer for badge/code-chip chat input

* feat: Add source-space undo/redo history for the rich input

* feat: Split text glued to a closing code fence onto its own line

* feat: Add ChatFormContenteditable rich input renderer

* feat : wire the contenteditable into ChatForm with auto-switch gating

tools/ui/src/lib/enums/keyboard.enums.ts

@@ -8,7 +8,6 @@ export enum KeyboardKey {
 	ARROW_DOWN = 'ArrowDown',
 	ARROW_LEFT = 'ArrowLeft',
 	ARROW_RIGHT = 'ArrowRight',
-	BACKSPACE = 'Backspace',
 	TAB = 'Tab',
 	B_LOWER = 'b',
 	D_LOWER = 'd',