feat(ui): enable storybook snapshot tests

- adds a process-compose namespace for running `storybook` and `luakit` together to replicate the `webkit`-based rendering that happens inside of `webview`
- adds some helper scripts for running storybook tests and updating snapshots, with documentation in the README.
- adds a `clan-app-ui-storybook` package which builds and tests the storybook, checking for rendering changes

Currently, we’re only doing markup-based snapshot tests. We’re also using headless chromium for the tests by default as I couldn't get webkit to work in the nix build.

As we’re only markup-based for the time being, this should be ok. But eventually I'd like to get it working with webkit.

pkgs/clan-app/flake-module.nix

@@ -29,6 +29,8 @@
         fonts = config.packages.fonts;
       };
 
+      packages.clan-app-ui-storybook = self'.packages.clan-app-ui.storybook;
+
       checks = config.packages.clan-app.tests;
     };
 }