Add foundational API client, UI components, and state management setup

- Created `generate-api-client.sh` for OpenAPI-based TypeScript client generation.
- Added `src/lib/api` with Axios-based API client, error handling utilities, and placeholder for generated types.
- Implemented Zustand-based `authStore` for user authentication and token management.
- Integrated reusable UI components (e.g., `Dialog`, `Select`, `Textarea`, `Sheet`, `Separator`, `Checkbox`) using Radix UI and utility functions.
- Established groundwork for client-server integration, state management, and modular UI development.

frontend/src/components/ui/checkbox.tsx

@@ -0,0 +1,29 @@
+"use client"
+
+import * as React from "react"
+import * as CheckboxPrimitive from "@radix-ui/react-checkbox"
+import { cn } from "@/lib/utils"
+import { CheckIcon } from "@radix-ui/react-icons"
+
+const Checkbox = React.forwardRef<
+  React.ElementRef<typeof CheckboxPrimitive.Root>,
+  React.ComponentPropsWithoutRef<typeof CheckboxPrimitive.Root>
+>(({ className, ...props }, ref) => (
+  <CheckboxPrimitive.Root
+    ref={ref}
+    className={cn(
+      "grid place-content-center peer h-4 w-4 shrink-0 rounded-sm border border-primary shadow focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-ring disabled:cursor-not-allowed disabled:opacity-50 data-[state=checked]:bg-primary data-[state=checked]:text-primary-foreground",
+      className
+    )}
+    {...props}
+  >
+    <CheckboxPrimitive.Indicator
+      className={cn("grid place-content-center text-current")}
+    >
+      <CheckIcon className="h-4 w-4" />
+    </CheckboxPrimitive.Indicator>
+  </CheckboxPrimitive.Root>
+))
+Checkbox.displayName = CheckboxPrimitive.Root.displayName
+
+export { Checkbox }