fix(app): contain triple-click selection to one message bubble

Triple-clicking a message extended the document Range across the
sibling composer and into the textarea, copying both on Cmd+C. Mark
the messages region non-selectable on web and re-enable selection on
each bubble's outer container so triple-click stays scoped.

packages/app/src/panels/agent-panel.tsx

@@ -14,7 +14,7 @@ import { FileDropZone } from "@/components/file-drop-zone";
 import type { ImageAttachment } from "@/components/message-input";
 import { getProviderIcon } from "@/components/provider-icons";
 import { ToastViewport, useToastHost } from "@/components/toast-host";
-import { isNative } from "@/constants/platform";
+import { isNative, isWeb } from "@/constants/platform";
 import { useAgentAttentionClear } from "@/hooks/use-agent-attention-clear";
 import { useAgentInitialization } from "@/hooks/use-agent-initialization";
 import { useAgentInputDraft } from "@/hooks/use-agent-input-draft";
@@ -1349,6 +1349,7 @@ const styles = StyleSheet.create((theme) => ({
   contentContainer: {
     flex: 1,
     overflow: "hidden",
+    ...(isWeb ? { userSelect: "none" as const } : {}),
   },
   content: {
     flex: 1,