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/checkbox.tsx

@@ -0,0 +1,31 @@
+import * as React from "react"
+import { Checkbox as CheckboxPrimitive } from "radix-ui"
+
+import { cn } from "@/lib/utils"
+import { CheckIcon } from "lucide-react"
+
+function Checkbox({
+  className,
+  ...props
+}: React.ComponentProps<typeof CheckboxPrimitive.Root>) {
+  return (
+    <CheckboxPrimitive.Root
+      data-slot="checkbox"
+      className={cn(
+        "peer relative flex size-4 shrink-0 items-center justify-center rounded-[4px] border border-input transition-colors outline-none group-has-disabled/field:opacity-50 after:absolute after:-inset-x-3 after:-inset-y-2 focus-visible:border-ring focus-visible:ring-3 focus-visible:ring-ring/50 disabled:cursor-not-allowed disabled:opacity-50 aria-invalid:border-destructive aria-invalid:ring-3 aria-invalid:ring-destructive/20 aria-invalid:aria-checked:border-primary dark:bg-input/30 dark:aria-invalid:border-destructive/50 dark:aria-invalid:ring-destructive/40 data-checked:border-primary data-checked:bg-primary data-checked:text-primary-foreground dark:data-checked:bg-primary",
+        className
+      )}
+      {...props}
+    >
+      <CheckboxPrimitive.Indicator
+        data-slot="checkbox-indicator"
+        className="grid place-content-center text-current transition-none [&>svg]:size-3.5"
+      >
+        <CheckIcon
+        />
+      </CheckboxPrimitive.Indicator>
+    </CheckboxPrimitive.Root>
+  )
+}
+
+export { Checkbox }