dsh-permission-guard: Add a permission settings page for DSH

In DSH, under Settings → General, there is only one line of permission configuration and no dedicated page. The `dsh-permission-guard` provides a "Permission Settings" page that allows toggling the default value for new sessions between "Full access" and "Workspace write" via buttons, and can elevate individual existing sessions to "Full access". It does not invent a permission model; instead, it writes to the `defaultPreset` in the permission namespace, performing a preset write similar to the `/permission` command. Before enabling "Full access", a confirmation prompt appears explaining the risks, which corresponds to `sandbox` being set to `danger-full-access` and the `approval policy` being set to `never`. It can be installed using `dsh plugin --profile desktop add` and requires DSH version 0.2.1-rc.5 or higher.