Set engine layout on ApplicationController

[fcatuhe]

Problem

When base_controller_class is configured to a controller that declares its own layout (e.g. layout "admin"), Litestream::ApplicationController inherits that layout. Since the host app's layout uses route helpers that aren't available inside a mounted engine, this causes a NameError:

NameError: undefined local variable or method 'some_host_app_path'
  from app/views/layouts/admin.html.erb:1

Root cause

Litestream::ApplicationController doesn't declare its own layout. The litestream/application layout file exists, but without an explicit layout declaration on the controller, Rails resolves the layout from the parent class.

Fix

Add layout "litestream/application" to Litestream::ApplicationController. This matches what other Rails engine dashboards do:

• mission_control-jobs: layout "mission_control/jobs/application"
• audits1984: layout "audits1984/application"
• blazer: layout "blazer/application"

Tests

• Unit test: asserts the controller's layout is litestream/application regardless of the base controller's layout
• Integration test: asserts the rendered page uses the engine's <title>Litestream</title>, not the host app's
• The dummy app's MyApplicationController now sets layout "application" to exercise this scenario