feat: implement MathTree MVP — skill tree, workbench, and progress system

- Interactive skill tree with dagre auto-layout and React Flow (15 math+logic nodes)
- Workbench with math input (LTR/RTL toggle), multiple choice, keyboard shortcuts
- Challenge verification engine with retry-based flow (no answer reveal until 3 failures)
- Scratchpad canvas with freehand drawing, text boxes, eraser, colors, and stroke sizes
- "Aprende primero" collapsible explanations on introductory challenges
- XP/level system, daily streaks, 7 achievements, progress persistence via Zustand
- Profile page with stats and achievement gallery
- Sidebar navigation with XP bar and streak badge

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/ui/separator.tsx

@@ -0,0 +1,25 @@
+"use client"
+
+import { Separator as SeparatorPrimitive } from "@base-ui/react/separator"
+
+import { cn } from "@/lib/utils"
+
+function Separator({
+  className,
+  orientation = "horizontal",
+  ...props
+}: SeparatorPrimitive.Props) {
+  return (
+    <SeparatorPrimitive
+      data-slot="separator"
+      orientation={orientation}
+      className={cn(
+        "shrink-0 bg-border data-horizontal:h-px data-horizontal:w-full data-vertical:w-px data-vertical:self-stretch",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+export { Separator }