fix(kez-chat/web): snapshot envelope before storing in IndexedDB

Save claim was silently failing — button click did nothing. Cause:
\`envelope\` lives in \$state, which wraps the value in a deep Proxy;
idb-keyval calls structuredClone internally, which can't clone proxies
and throws DataCloneError. Without a try/catch the error vanished into
the console and the step transition never ran.

- Pass \$state.snapshot(envelope) to addClaim so IDB sees a plain object.
- Wrap in try/catch + alert so future IDB failures surface to the user
  instead of dying silently.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

kez-chat/web/src/routes/AddClaim.svelte

@@ -180,12 +180,20 @@
 
   async function saveAndDone() {
     if (!envelope || !selected) return;
-    await addClaim({
-      id: crypto.randomUUID(),
-      envelope,
-      channel: selected.key,
-    });
-    step = "done";
+    try {
+      // $state wraps `envelope` in a deep Proxy; structuredClone (used
+      // by idb-keyval) can't clone proxies and throws DataCloneError.
+      // $state.snapshot returns a plain, cloneable object.
+      await addClaim({
+        id: crypto.randomUUID(),
+        envelope: $state.snapshot(envelope) as SignedClaimEnvelope,
+        channel: selected.key,
+      });
+      step = "done";
+    } catch (e) {
+      console.error("saveAndDone failed", e);
+      alert(`Failed to save claim: ${(e as Error).message}`);
+    }
   }
 </script>