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/ui.nix

@@ -2,11 +2,12 @@
   buildNpmPackage,
   nodejs_22,
   importNpmLock,
-
   clan-ts-api,
+  playwright-driver,
+  ps,
   fonts,
 }:
-buildNpmPackage {
+buildNpmPackage (finalAttrs: {
   pname = "clan-app-ui";
   version = "0.0.1";
   nodejs = nodejs_22;
@@ -15,6 +16,7 @@ buildNpmPackage {
   npmDeps = importNpmLock {
     npmRoot = ./ui;
   };
+
   npmConfigHook = importNpmLock.npmConfigHook;
 
   preBuild = ''
@@ -22,4 +24,36 @@ buildNpmPackage {
     cp -r ${clan-ts-api}/* api
     cp -r ${fonts} ".fonts"
   '';
-}
+
+  passthru = rec {
+    storybook = buildNpmPackage {
+      pname = "${finalAttrs.pname}-storybook";
+      inherit (finalAttrs)
+        version
+        nodejs
+        src
+        npmDeps
+        npmConfigHook
+        preBuild
+        ;
+
+      nativeBuildInputs = finalAttrs.nativeBuildInputs ++ [
+        ps
+      ];
+
+      npmBuildScript = "test-storybook-static";
+
+      env = finalAttrs.env // {
+        PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD = 1;
+        PLAYWRIGHT_BROWSERS_PATH = "${playwright-driver.browsers.override {
+          withChromiumHeadlessShell = true;
+        }}";
+        PLAYWRIGHT_HOST_PLATFORM_OVERRIDE = "ubuntu-24.04";
+      };
+
+      postBuild = ''
+        mv storybook-static $out
+      '';
+    };
+  };
+})