Update preview config from ticket system

Brings the Expo app onto main: the four screens from the design doc,
the design tokens, and the GLSL port of the dither background.

The branch was rooted while main was still unborn, so the two histories
are unrelated and the merge needs --allow-unrelated-histories. No files
overlap; main carried only .webklar-preview.json.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.webklar-preview.json

@@ -0,0 +1,11 @@
+{
+  "enabled": true,
+  "type": "static",
+  "branch": "main",
+  "displayName": "Brief-app",
+  "templateName": "webklar-preview-template",
+  "projectType": "preview",
+  "public": false,
+  "template": false,
+  "subdomain": "brief-app"
+}

package.json

@@ -31,7 +31,8 @@
     "android": "expo start --android",
     "ios": "expo start --ios",
     "web": "expo start --web",
-    "typecheck": "tsc --noEmit"
+    "typecheck": "tsc --noEmit",
+    "build": "expo export --platform web"
   },
   "private": true
 }