feat(viewer): second Vite entry, viewer capability, CodeMirror deps

Task 6 of the terminal file viewer plan: app/viewer.html plus
src/viewer/{main,ViewerApp}.tsx (placeholder ViewerApp for Task 11 to
replace), registers viewer.html as a second Rollup input in
vite.config.ts, adds the file-viewer capability restricted to
file-viewer-* windows (allow-listen/unlisten for the goto event,
allow-destroy for the close-button/prevent_close interaction,
allow-internal-toggle-devtools to match default.json's dev
convenience), installs the CodeMirror packages Task 10 builds
languages.ts on top of (P1), and pins the viewer entry/capability
with a Rust fallback-trap test.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

app/src/viewer/main.tsx

@@ -0,0 +1,10 @@
+import React from "react";
+import ReactDOM from "react-dom/client";
+import ViewerApp from "./ViewerApp";
+import "../index.css";
+
+ReactDOM.createRoot(document.getElementById("root")!).render(
+  <React.StrictMode>
+    <ViewerApp />
+  </React.StrictMode>,
+);