Refactor Zed/Flatpak options and add C++ & clangd settings

- Switch Zed remote server and Flatpak packages to use `enable`
suboption - Update host modules to match the new option hierarchy - Add
default C++ settings (format on save, tab size) to Zed user settings -
Introduce clangd LSP configuration with inlay hints and fallback command

modules/home/zed/user-settings/languages.nix

@@ -62,5 +62,9 @@ _: {
       };
       tab_size = 2;
     };
+    "C++" = {
+      format_on_save = "on";
+      tab_size = 2;
+    };
   };
 }