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/public/favicon.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64" fill="none">
+  <rect x="6" y="6" width="52" height="52" rx="4" stroke="#4a9eff" stroke-width="4" fill="#2d2d2d"/>
+  <rect x="14" y="14" width="36" height="36" rx="2" stroke="#4a9eff" stroke-width="2" fill="#1e1e1e"/>
+  <circle cx="26" cy="28" r="5" fill="#4a9eff" opacity="0.8"/>
+  <polygon points="20,44 32,30 38,36 44,28 44,44" fill="#4a9eff" opacity="0.6"/>
+</svg>