fix(agents): improve chat copying, selection and scrolling (#1905)

* fix(agents): improve chat transcript interactions

* fix(agents): recover images when regeneration fails

* test(agents): use the native copy shortcut on every host

src/styles/tokens.css

@@ -18,6 +18,7 @@ html {
   --space-2: 8px;
   --space-3: 12px;
   --space-4: 16px;
+  --agent-conversation-width: 900px;
   --splash-logo-size: 56px;
   --splash-bar-width: 120px;
   --splash-bar-height: 2px;