fix(frontend): fix step numbering and client-side API base URL

- Step numbers now use each-block index (i+1) instead of order_index+1,
  fixing display when order_index starts from 1 in existing data
- api.ts: browser-side requests use /api (nginx proxy) instead of
  PUBLIC_API_BASE (localhost:8000), fixing PATCH/client calls on remote hosts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/api.ts

@@ -1,3 +1,4 @@
+import { browser } from '$app/environment';
 import { PUBLIC_API_BASE } from '$env/static/public';
 import type {
 	ActiveIngredient,
@@ -21,7 +22,10 @@ import type {
 // ─── Core fetch helpers ──────────────────────────────────────────────────────
 
 async function request<T>(path: string, init: RequestInit = {}): Promise<T> {
-	const url = `${PUBLIC_API_BASE}${path}`;
+	// Server-side uses PUBLIC_API_BASE (e.g. http://localhost:8000).
+	// Browser-side uses /api so nginx proxies the request on the correct host.
+	const base = browser ? '/api' : PUBLIC_API_BASE;
+	const url = `${base}${path}`;
 	const res = await fetch(url, {
 		headers: { 'Content-Type': 'application/json', ...init.headers },
 		...init