Allow assigning applications to a group from the group form

Adds an "Assigned Applications" checkbox list to the group new/edit
form so admins can grant a group access to multiple apps from one
screen, instead of editing each application individually.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/controllers/admin/groups_controller_test.rb

@@ -0,0 +1,52 @@
+require "test_helper"
+
+module Admin
+  class GroupsControllerTest < ActionDispatch::IntegrationTest
+    setup do
+      @admin = users(:two)
+      sign_in_as(@admin)
+      @group = groups(:one)
+    end
+
+    test "update assigns applications from application_ids" do
+      app_a = applications(:kavita_app)
+      app_b = applications(:another_app)
+
+      patch admin_group_path(@group), params: {
+        group: {
+          name: @group.name,
+          application_ids: [app_a.id, app_b.id]
+        }
+      }
+
+      assert_redirected_to admin_group_path(@group)
+      assert_equal [app_a, app_b].sort, @group.reload.applications.sort
+    end
+
+    test "update with no application_ids clears assigned applications" do
+      @group.applications = [applications(:kavita_app)]
+
+      patch admin_group_path(@group), params: {
+        group: { name: @group.name }
+      }
+
+      assert_redirected_to admin_group_path(@group)
+      assert_empty @group.reload.applications
+    end
+
+    test "create assigns applications from application_ids" do
+      app = applications(:audiobookshelf_app)
+
+      assert_difference -> { Group.count }, 1 do
+        post admin_groups_path, params: {
+          group: {
+            name: "New Group",
+            application_ids: [app.id]
+          }
+        }
+      end
+
+      assert_equal [app], Group.find_by(name: "new group").applications
+    end
+  end
+end