feat: add deployment setting to disable the Codernauts game (#27662) - #28901

Adds a `codernauts_enabled` runtime setting (default: enabled) that
controls whether the Codernauts game link appears in the user dropdown
menu. A toggle at the bottom of the Deployment > Appearance page
controls it, and the value persists in `site_configs`, so it survives
restarts and upgrades without any CLI flags or environment variables.

The setting works on all licenses: `GET /api/v2/appearance` reports it
from both the enterprise and the default (AGPL) appearance fetchers, and
`PUT /api/v2/appearance` persists it gated only by the deployment-config
RBAC permission, not by an entitlement. On the Appearance page the
toggle renders outside the Premium paywall introduced in #27948; the
other appearance fields keep their existing licensing behavior.

Note: a pure AGPL-only build of coderd does not register the
`/api/v2/appearance` routes at all, so the toggle cannot be saved there.
The standard `coder server` binary is unaffected regardless of license
state.

<details>
<summary>Implementation notes</summary>

- `coderd/database/queries/siteconfig.sql`: new `GetCodernautsEnabled` /
`UpsertCodernautsEnabled` queries backed by a `codernauts_enabled` key
in `site_configs`; defaults to `true` when unset (no migration needed).
- `coderd/database/dbauthz`: read has no authz checks (matching other
appearance reads); write requires `ResourceDeploymentConfig` update.
Coverage added to `dbauthz_test.go`.
- `codersdk`: `codernauts_enabled` added to `AppearanceConfig` and
`UpdateAppearanceConfig`.
- `coderd/appearance`: the default fetcher takes a `database.Store` and
reads the setting from the database so unlicensed deployments serve it
too.
- `enterprise/coderd/appearance.go`: fetches the value in `Fetch` and
persists it in `putAppearance`.
- Frontend: "Codernauts game" toggle section on Deployment > Appearance
saving immediately via the existing appearance mutation;
`codernauts_enabled` threaded from `useDashboard().appearance` through
Navbar to `UserDropdownContent`, which renders the menu item only when
enabled. Storybook stories with `play` functions cover toggling
(entitled and not) and the dropdown hiding the link when disabled.

</details>

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🤖 This PR was generated by Coder Agents on behalf of @bartekgatzcoder.

(cherry picked from commit 55340de)