fix: QR menu branding fields — display name, header image, empty-to-hide

- menu_display_name is now separate from the site's internal name (was
  incorrectly showing the sysadmin-only site name on the public menu).
- Header image now resolves against the cloud API origin instead of
  the menu-app's own origin, fixing the broken-image icon.
- Tagline/blurb/hours now genuinely hide when cleared: the sysadmin
  save handler was converting empty fields to null, which the PUT
  endpoint's exclude_none silently drops instead of clearing.
- Added a settable blurb (EN/GR) below the tagline, previously
  hardcoded ("Fresh seasonal plates, served with care.").
- Collapsed hours from two fields (EN/GR) to one — opening hours don't
  vary by language.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

connect_frontend/menu-app/src/api.js

@@ -1,8 +1,8 @@
 import axios from 'axios'
 
-const BASE = import.meta.env.VITE_CLOUD_URL || ''
+export const CLOUD_URL = import.meta.env.VITE_CLOUD_URL || ''
 
-const api = axios.create({ baseURL: BASE })
+const api = axios.create({ baseURL: CLOUD_URL })
 
 export async function fetchMenu(siteSlug) {
   const { data } = await api.get(`/api/menu/${siteSlug}`)