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/index.css

@@ -0,0 +1,49 @@
+*, *::before, *::after {
+  box-sizing: border-box;
+}
+
+html, body, #root {
+  margin: 0;
+  padding: 0;
+  width: 100%;
+  height: 100%;
+  background: #1a1a1a;
+  color: #e0e0e0;
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
+}
+
+a {
+  color: #4a9eff;
+  text-decoration: none;
+}
+
+a:hover {
+  text-decoration: underline;
+}
+
+input, button, select, textarea {
+  font-family: inherit;
+  font-size: inherit;
+}
+
+button {
+  cursor: pointer;
+}
+
+::-webkit-scrollbar {
+  width: 8px;
+  height: 8px;
+}
+
+::-webkit-scrollbar-track {
+  background: #1a1a1a;
+}
+
+::-webkit-scrollbar-thumb {
+  background: #3d3d3d;
+  border-radius: 4px;
+}
+
+::-webkit-scrollbar-thumb:hover {
+  background: #555;
+}