feat: implement dynamic sidebar with server-side menu translation

- Add server-side translation for menu labels using user's preferred_language
- Replace hardcoded sidebar template with dynamic rendering from menu API
- Remove hardcoded section/page mappings in favor of menu discovery
- Fix locale file structure (move menu keys to top level to avoid double-nesting)
- Sidebar now fully driven by /admin/menu-config/render/admin API endpoint

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/modules/marketplace/locales/de.json

@@ -1,4 +1,12 @@
 {
+  "menu": {
+    "marketplace": "Marktplatz",
+    "letzshop": "Letzshop",
+    "products_inventory": "Produkte & Inventar",
+    "marketplace_import": "Marktplatz Import",
+    "sales_orders": "Verkäufe & Bestellungen",
+    "letzshop_orders": "Letzshop Bestellungen"
+  },
   "marketplace": {
     "title": "Marktplatz",
     "import": "Importieren",

app/modules/marketplace/locales/en.json

@@ -1,4 +1,12 @@
 {
+  "menu": {
+    "marketplace": "Marketplace",
+    "letzshop": "Letzshop",
+    "products_inventory": "Products & Inventory",
+    "marketplace_import": "Marketplace Import",
+    "sales_orders": "Sales & Orders",
+    "letzshop_orders": "Letzshop Orders"
+  },
   "marketplace": {
     "title": "Marketplace",
     "import": "Import",

app/modules/marketplace/locales/fr.json

@@ -1,4 +1,12 @@
 {
+  "menu": {
+    "marketplace": "Marketplace",
+    "letzshop": "Letzshop",
+    "products_inventory": "Produits et Inventaire",
+    "marketplace_import": "Import Marketplace",
+    "sales_orders": "Ventes et Commandes",
+    "letzshop_orders": "Commandes Letzshop"
+  },
   "marketplace": {
     "title": "Marketplace",
     "import": "Importer",

app/modules/marketplace/locales/lb.json

@@ -1,4 +1,12 @@
 {
+  "menu": {
+    "marketplace": "Marchéplaz",
+    "letzshop": "Letzshop",
+    "products_inventory": "Produkter & Inventar",
+    "marketplace_import": "Marchéplaz Import",
+    "sales_orders": "Verkaf & Bestellungen",
+    "letzshop_orders": "Letzshop Bestellungen"
+  },
   "marketplace": {
     "title": "Marchéplaz",
     "import": "Import",