feat: add web dashboard for policy visualization and management

Debugging policies is currently difficult because we have to manually check logs and crd statuses. Editing yaml directly is also error-prone. To improve this, we need a built-in web dashboard similar to opa gatekeeper's tools.

- **Visualization (like policy reporter):** We need a simple overview page. It should show basic metrics from prometheus, such as total request counts and violation rates, so we can see the cluster status at a glance.
- **Policy editor (like gatekeeper policy manager):** Writing admissionpolicy yaml by hand causes too many syntax errors. A graphical form is needed to edit rules safely and validate inputs before applying them.
- **Playground (dry run):** Testing rules on a live cluster is risky. We need a "playground" tab where we can paste a json request and test it against a policy using the existing casbin enforcer, without affecting the real environment.

Please implement this as a subcommand in the existing go binary (e.g., policywall dashboard). It should serve static html templates directly. Do not create a separate backend service.