feat(04-01): implement MarketplaceCard component and marketplace i18n namespace

- Add marketplace namespace to de.json and en.json with all copywriting contract strings
- Add sidebar.search and sidebar.noResults i18n keys
- Create MarketplaceCard with icon, name, localized description, category badge,
  status badge (role="status"), and activate/deactivate button
- All 5 unit tests pass (GREEN phase)

apps/web/src/messages/en.json

@@ -81,6 +81,8 @@
     "collapse": "Collapse",
     "expand": "Expand",
     "userPlaceholder": "User",
+    "search": "Search modules...",
+    "noResults": "No modules found",
     "categories": {
       "label": "Categories",
       "empty": "No modules"
@@ -171,5 +173,34 @@
     "noResults": "Enter a domain name to check availability",
     "checking": "Checking...",
     "error": "Error checking domain"
+  },
+  "marketplace": {
+    "title": "Marketplace",
+    "subtitle": "Discover and activate modules for your tenant",
+    "activate": "Activate Module",
+    "deactivate": "Deactivate",
+    "searchPlaceholder": "Search modules...",
+    "statusAll": "All",
+    "statusActive": "Activated",
+    "statusAvailable": "Available",
+    "categoryAll": "All",
+    "emptyTitle": "No modules available yet",
+    "emptyBody": "Modules are provided by the system administrator. Check back later.",
+    "filteredEmptyTitle": "No results",
+    "filteredEmptyBody": "Try a different search term or adjust your filters.",
+    "errorLoad": "Could not load modules. Please reload the page.",
+    "toastActivated": "Module activated successfully",
+    "toastDeactivated": "Module deactivated successfully",
+    "toastActivateError": "Activation failed. Please try again.",
+    "toastDeactivateError": "Deactivation failed. Please try again.",
+    "deactivateDialogTitle": "Deactivate Module",
+    "deactivateDialogBody": "Do you want to deactivate {moduleName} for this tenant? The module will be removed from the sidebar.",
+    "deactivateDialogConfirm": "Deactivate",
+    "deactivateDialogCancel": "Cancel",
+    "tenantContextLabel": "Tenant Context",
+    "backToMarketplace": "Back to Marketplace",
+    "detailVersion": "Version {version}",
+    "detailStatusActive": "Activated for this tenant",
+    "detailStatusInactive": "Not activated"
   }
 }