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/sonner.tsx

@@ -0,0 +1,31 @@
+"use client"
+
+import { useTheme } from "next-themes"
+import { Toaster as Sonner } from "sonner"
+
+type ToasterProps = React.ComponentProps<typeof Sonner>
+
+const Toaster = ({ ...props }: ToasterProps) => {
+  const { theme = "system" } = useTheme()
+
+  return (
+    <Sonner
+      theme={theme as ToasterProps["theme"]}
+      className="toaster group"
+      toastOptions={{
+        classNames: {
+          toast:
+            "group toast group-[.toaster]:bg-background group-[.toaster]:text-foreground group-[.toaster]:border-border group-[.toaster]:shadow-lg",
+          description: "group-[.toast]:text-muted-foreground",
+          actionButton:
+            "group-[.toast]:bg-primary group-[.toast]:text-primary-foreground",
+          cancelButton:
+            "group-[.toast]:bg-muted group-[.toast]:text-muted-foreground",
+        },
+      }}
+      {...props}
+    />
+  )
+}
+
+export { Toaster }