Add 1280x800 store listing screenshots + composition harness

- store/store-1-dashboard.png: findings console with sample data
- store/store-2-popup.png: hero composition with popup UI
- store/store-3-empty.png: radar empty state
- preview/store.css + store-popup.html: composition page
- preview/results-preview-empty.html + preview-stub-empty.js: empty-state variant

preview/preview-stub.js

@@ -29,7 +29,7 @@
 
   window.chrome = {
     runtime: {
-      getManifest: () => ({ version: "2.1.1" }),
+      getManifest: () => ({ version: "2.2.0" }),
       sendMessage: async (msg) => {
         switch (msg && msg.type) {
           case "getFindings": return { findings: SAMPLE_FINDINGS.slice() };