Add Ctrl+K/Cmd+K command palette search

Replace the TopBar dropdown search with a cmdk-based command palette opened via a visible trigger button (with platform-aware shortcut badge) or the Ctrl+K/Cmd+K global shortcut. Shared search logic moves to a useGlobalSearch hook; the palette reuses the existing ui/command components.

src/i18n/en.json

@@ -222,7 +222,10 @@
     "list": "List",
     "noResults": "No results found.",
     "pages": "Pages",
-    "settings": "Settings"
+    "placeholder": "Search pages, fields, settings...",
+    "settings": "Settings",
+    "title": "Search",
+    "typeToSearch": "Type to search the admin panel."
   },
   "jmapErrors": {
     "addressBookHasContents": "This address book has contacts. Remove them first.",