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/vite.config.ts

@@ -1,3 +1,4 @@
+import { fileURLToPath } from "node:url";
 import { defineConfig } from "vite";
 import react from "@vitejs/plugin-react";
 import tailwindcss from "@tailwindcss/vite";
@@ -22,4 +23,12 @@ export default defineConfig({
       ignored: ["**/src-tauri/**"],
     },
   },
+  build: {
+    rollupOptions: {
+      input: {
+        main: fileURLToPath(new URL("index.html", import.meta.url)),
+        viewer: fileURLToPath(new URL("viewer.html", import.meta.url)),
+      },
+    },
+  },
 });