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/utils/source-history.ts

@@ -0,0 +1,48 @@
+/**
+ * Source-space undo/redo history for the chat-form contenteditable, whose
+ * imperative DOM rebuilds destroy the browser's native undo stack.
+ * Entries record the state BEFORE an edit; edits within `groupWindowMs`
+ * extend the open group so a typing burst undoes as a unit, while
+ * structural edits (paste, mention insert, clear) pass `newGroup`.
+ */
+
+export interface SourceHistoryEntry {
+	value: string;
+	caret: number;
+}
+
+export class SourceHistory {
+	private undoStack: SourceHistoryEntry[] = [];
+	private redoStack: SourceHistoryEntry[] = [];
+	private lastPush = 0;
+
+	constructor(
+		private limit = 100,
+		private groupWindowMs = 800
+	) {}
+
+	push(entry: SourceHistoryEntry, now: number, newGroup = false): void {
+		if (newGroup || now - this.lastPush >= this.groupWindowMs || this.undoStack.length === 0) {
+			this.undoStack.push(entry);
+			if (this.undoStack.length > this.limit) this.undoStack.shift();
+		}
+		this.lastPush = now;
+		this.redoStack = [];
+	}
+
+	undo(current: SourceHistoryEntry): SourceHistoryEntry | null {
+		const entry = this.undoStack.pop();
+		if (!entry) return null;
+		this.redoStack.push(current);
+		this.lastPush = 0; // the next edit after an undo starts a new group
+		return entry;
+	}
+
+	redo(current: SourceHistoryEntry): SourceHistoryEntry | null {
+		const entry = this.redoStack.pop();
+		if (!entry) return null;
+		this.undoStack.push(current);
+		this.lastPush = 0;
+		return entry;
+	}
+}