feat: per-site QR menu mode (order vs view-only) + editable branding

Adds a sysadmin-configurable toggle so sites can disable online ordering
on the public QR menu until it's fully supported, plus editable
tagline/hours and a header image (replacing the hardcoded "Our Menu"
placeholder) — all previously hardcoded frontend strings.

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

cloud_backend/routers/menu.py

@@ -36,7 +36,15 @@ def get_menu(site_slug: str, db: Session = Depends(get_db)):
         raise HTTPException(status_code=404, detail="No menu published yet")
 
     import json
-    return json.loads(snapshot.snapshot_json)
+    data = json.loads(snapshot.snapshot_json)
+    data["menu_mode"] = site.menu_mode
+    data["restaurant"] = {
+        "name": site.name,
+        "tagline": {"en": site.menu_tagline_en, "gr": site.menu_tagline_gr},
+        "hours": {"en": site.menu_hours_en, "gr": site.menu_hours_gr},
+        "headerImageUrl": site.menu_header_image_url,
+    }
+    return data
 
 
 # ── Internal (site API key) ───────────────────────────────────────────────────