feat(phase-13): implement layer groups/folders system

Add comprehensive layer groups system for organizing layers hierarchically.

Features:
- Create layer groups (folders)
- Add layers to groups
- Remove layers from groups
- Toggle group collapsed/expanded state
- Get all layers in a group
- Group properties:
  * groupId: Parent group ID (null if not in group)
  * isGroup: Whether layer is a group
  * collapsed: Whether group is collapsed
- Groups are special layers with isGroup=true
- Groups have no canvas (width/height = 0)
- Groups can contain multiple layers
- Layers track their parent group via groupId

Changes:
- Updated types/layer.ts with group properties:
  * groupId: string | null
  * isGroup: boolean
  * collapsed: boolean
- Updated store/layer-store.ts:
  * createLayer initializes group properties
  * createGroup() - Create new group
  * addToGroup() - Add layer to group
  * removeFromGroup() - Remove from group
  * toggleGroupCollapsed() - Toggle collapsed
  * getGroupLayers() - Get group's layers

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

types/layer.ts

@@ -59,6 +59,12 @@ export interface Layer {
   y: number;
   /** Layer mask for non-destructive editing */
   mask: LayerMask | null;
+  /** Parent group ID (null if not in a group) */
+  groupId: string | null;
+  /** Whether this layer is a group */
+  isGroup: boolean;
+  /** Whether group is collapsed (only relevant if isGroup=true) */
+  collapsed: boolean;
   /** Timestamp of creation */
   createdAt: number;
   /** Timestamp of last modification */