feat(frontend): add Front/ — Vite/React frontend with backend pipeline integration

Mirror of design_agent_front/design-agent/ for shipping alongside backend.

Vite plugin (vitePluginPhaseZApi) endpoints :
  - POST /api/run   — spawn `python -m src.phase_z2_pipeline` with overrides
  - GET  /api/sample-mdx?mdx=03/04/05 — fixed sample MDX
  - GET  /frame-preview/{n} — figma preview thumbnails
  - GET  /data/runs/{run_id}/{path} — pipeline artifacts (final.html, step*.json, ...)

Env toggle forward (보고용) :
  PHASE_Z_ALLOW_RESTRUCTURE / PHASE_Z_ALLOW_REJECT / PHASE_Z_MAX_RANK=32

Components :
  - LeftMdxPanel (03/04/05 fix list + section tree)
  - SlideCanvas (iframe + slideOverrideCss prop for inline CSS inject)
  - FramePanel (label priority + confidence sort)
  - LayoutPanel

README with mermaid diagrams covering the 5-step demo flow.
node_modules / dist / .manus-logs / .env excluded via .gitignore.

Front/client/src/components/ui/button-group.tsx

@@ -0,0 +1,83 @@
+import { Slot } from "@radix-ui/react-slot";
+import { cva, type VariantProps } from "class-variance-authority";
+
+import { cn } from "@/lib/utils";
+import { Separator } from "@/components/ui/separator";
+
+const buttonGroupVariants = cva(
+  "flex w-fit items-stretch [&>*]:focus-visible:z-10 [&>*]:focus-visible:relative [&>[data-slot=select-trigger]:not([class*='w-'])]:w-fit [&>input]:flex-1 has-[select[aria-hidden=true]:last-child]:[&>[data-slot=select-trigger]:last-of-type]:rounded-r-md has-[>[data-slot=button-group]]:gap-2",
+  {
+    variants: {
+      orientation: {
+        horizontal:
+          "[&>*:not(:first-child)]:rounded-l-none [&>*:not(:first-child)]:border-l-0 [&>*:not(:last-child)]:rounded-r-none",
+        vertical:
+          "flex-col [&>*:not(:first-child)]:rounded-t-none [&>*:not(:first-child)]:border-t-0 [&>*:not(:last-child)]:rounded-b-none",
+      },
+    },
+    defaultVariants: {
+      orientation: "horizontal",
+    },
+  }
+);
+
+function ButtonGroup({
+  className,
+  orientation,
+  ...props
+}: React.ComponentProps<"div"> & VariantProps<typeof buttonGroupVariants>) {
+  return (
+    <div
+      role="group"
+      data-slot="button-group"
+      data-orientation={orientation}
+      className={cn(buttonGroupVariants({ orientation }), className)}
+      {...props}
+    />
+  );
+}
+
+function ButtonGroupText({
+  className,
+  asChild = false,
+  ...props
+}: React.ComponentProps<"div"> & {
+  asChild?: boolean;
+}) {
+  const Comp = asChild ? Slot : "div";
+
+  return (
+    <Comp
+      className={cn(
+        "bg-muted flex items-center gap-2 rounded-md border px-4 text-sm font-medium shadow-xs [&_svg]:pointer-events-none [&_svg:not([class*='size-'])]:size-4",
+        className
+      )}
+      {...props}
+    />
+  );
+}
+
+function ButtonGroupSeparator({
+  className,
+  orientation = "vertical",
+  ...props
+}: React.ComponentProps<typeof Separator>) {
+  return (
+    <Separator
+      data-slot="button-group-separator"
+      orientation={orientation}
+      className={cn(
+        "bg-input relative !m-0 self-stretch data-[orientation=vertical]:h-auto",
+        className
+      )}
+      {...props}
+    />
+  );
+}
+
+export {
+  ButtonGroup,
+  ButtonGroupSeparator,
+  ButtonGroupText,
+  buttonGroupVariants,
+};