feat: add design persistence functionality with Auth0 and Supabase integration

- Implemented `useDesignPersistence` composable for managing design records.
- Enhanced `useSlipmatDesigner` to support loading designs from JSON.
- Created global authentication middleware for route protection.
- Added Supabase client plugin for database interactions.
- Developed API endpoints for fetching, saving, and retrieving designs.
- Introduced utility functions for Auth0 token verification and Supabase client retrieval.
- Updated Nuxt configuration to include Auth0 and Supabase environment variables.
- Added necessary dependencies for Auth0 and Supabase.
- Enhanced TypeScript configuration for improved type support.

server/api/designs/[id].get.ts

@@ -0,0 +1,47 @@
+import { createError } from "h3";
+
+import { requireAuth0User } from "../../utils/auth0";
+import { getSupabaseServiceClient } from "../../utils/supabase";
+
+type DesignRecord = {
+  id: string;
+  user_id: string;
+  name: string;
+  template_id: string;
+  preview_url: string | null;
+  preview_path: string | null;
+  design_json: unknown;
+  notes?: string | null;
+  created_at: string | null;
+  updated_at: string | null;
+};
+
+export default defineEventHandler(async (event) => {
+  const user = await requireAuth0User(event);
+  const supabase = getSupabaseServiceClient();
+  const id = event.context.params?.id;
+
+  if (!id) {
+    throw createError({ statusCode: 400, statusMessage: "Design id is required." });
+  }
+
+  const response = await supabase
+    .from("designs")
+    .select("id, user_id, name, template_id, preview_url, preview_path, design_json, notes, created_at, updated_at")
+    .eq("id", id)
+    .eq("user_id", user.sub)
+    .maybeSingle();
+
+  if (response.error) {
+    throw createError({
+      statusCode: 500,
+      statusMessage: `Failed to load design: ${response.error.message}`,
+    });
+  }
+
+  if (!response.data) {
+    throw createError({ statusCode: 404, statusMessage: "Design not found." });
+  }
+
+  return response.data as DesignRecord;
+});