Replace Firebase Storage with MinIO and add user account features

- Storage Integration:
  * Remove Firebase Storage dependency and useFirebaseStorage composable
  * Implement direct MinIO uploads via POST /storage/upload with multipart/form-data
  * Upload canvas JSON, preview PNG, and production PNG as separate objects
  * Store public URLs and metadata in design records

- Authentication & Registration:
  * Add email/password registration page with validation
  * Integrate backend user session via /auth/login endpoint
  * Store backendUser.id as ownerId in design records
  * Auto-sync backend session on Firebase auth state changes

- User Account Pages:
  * Create profile page showing user details and backend session info
  * Create orders page with transaction history filtered by customerEmail
  * Add server proxy /api/orders to forward GET /transactions queries

- Navigation Improvements:
  * Replace inline auth buttons with avatar dropdown menu
  * Add Profile, Orders, and Logout options to dropdown
  * Implement outside-click and route-change handlers for dropdown
  * Display user initials in avatar badge

- API Updates:
  * Update transactions endpoint to accept amount as string
  * Format amount with .toFixed(2) in checkout success flow
  * Query orders by customerEmail instead of ownerId for consistency

server/api/designs.post.ts

@@ -0,0 +1,60 @@
+export default defineEventHandler(async (event) => {
+  const body = await readBody<{
+    designId: string;
+    templateId: string;
+    ownerEmail?: string | null;
+    ownerId?: string | null;
+    previewUrl?: string | null;
+    productionUrl?: string | null;
+    canvasJson: unknown;
+    metadata?: Record<string, unknown>;
+  }>(event);
+
+  if (!body?.designId || !body?.templateId || body.canvasJson === undefined) {
+    throw createError({
+      statusCode: 400,
+      statusMessage: "Missing required design fields",
+    });
+  }
+
+  const config = useRuntimeConfig();
+  const backendUrl = config.public?.backendUrl;
+
+  if (!backendUrl) {
+    throw createError({
+      statusCode: 500,
+      statusMessage: "Backend URL not configured",
+    });
+  }
+
+  const payload = {
+    designId: body.designId,
+    templateId: body.templateId,
+    ownerEmail: body.ownerEmail ?? null,
+    ownerId: body.ownerId ?? null,
+    previewUrl: body.previewUrl ?? null,
+    productionUrl: body.productionUrl ?? null,
+    canvasJson: body.canvasJson,
+    metadata: body.metadata ?? {},
+    updatedAt: new Date().toISOString(),
+  };
+
+  try {
+    const result = await $fetch("/designs", {
+      baseURL: backendUrl,
+      method: "POST",
+      body: payload,
+    });
+
+    return {
+      ok: true,
+      result,
+    };
+  } catch (err) {
+    console.error("[designs] Failed to forward design payload", err);
+    throw createError({
+      statusCode: 502,
+      statusMessage: (err as Error)?.message ?? "Failed to persist design",
+    });
+  }
+});