feat: add SPA, serve at /app/, update Dockerfile and README

- React + TanStack Router + shadcn/ui SPA under spa/
- serve spa/dist at /app/ with index.html fallback for client routing
- Dockerfile: node build stage for SPA, copy dist into runtime image
- README: document SPA, CORS_ORIGINS env var, architecture entry
- vite base set to /app/, manifest.json paths fixed

spa/src/components/ui/progress.tsx

@@ -0,0 +1,31 @@
+"use client"
+
+import * as React from "react"
+import { Progress as ProgressPrimitive } from "radix-ui"
+
+import { cn } from "@/lib/utils"
+
+function Progress({
+  className,
+  value,
+  ...props
+}: React.ComponentProps<typeof ProgressPrimitive.Root>) {
+  return (
+    <ProgressPrimitive.Root
+      data-slot="progress"
+      className={cn(
+        "relative flex h-1 w-full items-center overflow-x-hidden rounded-full bg-muted",
+        className
+      )}
+      {...props}
+    >
+      <ProgressPrimitive.Indicator
+        data-slot="progress-indicator"
+        className="size-full flex-1 bg-primary transition-all"
+        style={{ transform: `translateX(-${100 - (value || 0)}%)` }}
+      />
+    </ProgressPrimitive.Root>
+  )
+}
+
+export { Progress }