feat: RefBoard v0.4.0 — collaborative reference board with layers, groups & polished UI

Full-featured PureRef-style collaborative canvas for game dev teams:
- Layer panel with visibility, lock, drag reorder, group/ungroup (Ctrl+G/Shift+G)
- Arrangement tools (grid, row, column) via right-click context menu
- Copy to system clipboard (Ctrl+C writes PNG for external paste in Paint etc.)
- Number shortcuts (1-5) for tool selection with visible shortcut badges
- Premium dark UI across all pages (Login, Collections, Boards, Editor)
- Socket.IO rooms for cursors, transforms, and presence notifications
- MinIO image storage with backend proxy, drag/drop and paste upload

frontend/src/canvas/history.ts

@@ -0,0 +1,83 @@
+import { Canvas } from 'fabric';
+
+export class UndoManager {
+  private stack: string[] = [];
+  private pointer: number = -1;
+  private maxEntries: number = 50;
+  private locked: boolean = false;
+  private canvas: Canvas;
+
+  constructor(canvas: Canvas) {
+    this.canvas = canvas;
+    // Save initial state
+    this.saveState();
+  }
+
+  isLocked(): boolean {
+    return this.locked;
+  }
+
+  saveState(): void {
+    if (this.locked) return;
+
+    const json = JSON.stringify((this.canvas as any).toJSON(['id']));
+
+    // If we're not at the end, discard forward history
+    if (this.pointer < this.stack.length - 1) {
+      this.stack = this.stack.slice(0, this.pointer + 1);
+    }
+
+    // Don't save if identical to current state
+    if (this.stack.length > 0 && this.stack[this.pointer] === json) {
+      return;
+    }
+
+    this.stack.push(json);
+
+    // Enforce max entries
+    if (this.stack.length > this.maxEntries) {
+      this.stack.shift();
+    }
+
+    this.pointer = this.stack.length - 1;
+  }
+
+  undo(): void {
+    if (!this.canUndo()) return;
+
+    this.pointer--;
+    this.restoreState();
+  }
+
+  redo(): void {
+    if (!this.canRedo()) return;
+
+    this.pointer++;
+    this.restoreState();
+  }
+
+  canUndo(): boolean {
+    return this.pointer > 0;
+  }
+
+  canRedo(): boolean {
+    return this.pointer < this.stack.length - 1;
+  }
+
+  private restoreState(): void {
+    const state = this.stack[this.pointer];
+    if (!state) return;
+
+    this.locked = true;
+    this.canvas.loadFromJSON(JSON.parse(state)).then(() => {
+      this.canvas.requestRenderAll();
+      this.locked = false;
+    });
+  }
+
+  clear(): void {
+    this.stack = [];
+    this.pointer = -1;
+    this.saveState();
+  }
+}