fix: reject contradictory create/update config

[tfwright]

Summary

Validates at compile time, in admin_resource/2, that a resource isn't configured with create_with: false alongside a custom :create component (or update_with: false alongside a custom :edit component). These combinations are contradictory: the disabling option says the action is off, while the component override says it's customized.

Catching this at compile time forces users to pick one approach rather than the silent / unexpected runtime behavior reported in #139.

Implementation notes:

• live_admin/3 stashes scope opts in @__live_admin_scope_opts__ on the router module.
• admin_resource/2 calls LiveAdmin.Router.__validate_config__!/3 at compile time with the resource module, scope opts, and the app-level config read via Application.compile_env.
• App-level :create_with, :update_with, and :components are now read via compile_env rather than get_env. They can no longer be changed at runtime without recompiling.

Closes #139

Test plan

• docker compose run app mix test — 39 tests pass
• docker compose run app mix credo on changed files — clean
• New test/live_admin/router_test.exs covers: resource-level conflict, scope-level conflict, app-level conflict, and a no-conflict baseline