Add user feedback improvements: server error reporting, Clarity analytics, usage survey

- Auto-report link on 5xx/network errors: pre-fills GitHub issue with version, error, and stylesheet snippet
- Microsoft Clarity: conditional on VITE_CLARITY_ID env var, loaded after page load
- Usage survey: appears after 3 successful transforms, fires GA4 event, one-shot (stored in localStorage)
- 27 tests passing (10 new tests covering buildBugReportUrl, server error detection, survey lifecycle)

frontend/index.html

@@ -103,6 +103,17 @@
       }
     </style>
     <script type="module" src="/env.js"></script>
+    <script>
+      window.addEventListener("load", function () {
+        var clarityId = window.env && window.env.VITE_CLARITY_ID;
+        if (!clarityId) return;
+        (function (c, l, a, r, i, t, y) {
+          c[a] = c[a] || function () { (c[a].q = c[a].q || []).push(arguments); };
+          t = l.createElement(r); t.async = 1; t.src = "https://www.clarity.ms/tag/" + i;
+          y = l.getElementsByTagName(r)[0]; y.parentNode.insertBefore(t, y);
+        })(window, document, "clarity", "script", clarityId);
+      }, { once: true });
+    </script>
     <script>
       const loadAdsense = () => {
         const client = window.env && window.env.VITE_ADSENSE_CLIENT;