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/package.json

@@ -13,6 +13,21 @@
     "hooks": "git -C .. config core.hooksPath .githooks && echo \"pre-commit secret scan enabled\""
   },
   "dependencies": {
+    "@codemirror/commands": "^6.11.1",
+    "@codemirror/lang-css": "^6.3.1",
+    "@codemirror/lang-html": "^6.4.12",
+    "@codemirror/lang-javascript": "^6.2.5",
+    "@codemirror/lang-json": "^6.0.2",
+    "@codemirror/lang-markdown": "^6.5.2",
+    "@codemirror/lang-python": "^6.2.1",
+    "@codemirror/lang-rust": "^6.0.2",
+    "@codemirror/lang-yaml": "^6.1.3",
+    "@codemirror/language": "^6.12.4",
+    "@codemirror/legacy-modes": "^6.5.4",
+    "@codemirror/search": "^6.7.2",
+    "@codemirror/state": "^6.7.6",
+    "@codemirror/view": "^6.43.13",
+    "@lezer/highlight": "^1.2.3",
     "@tauri-apps/api": "^2",
     "@tauri-apps/plugin-dialog": "^2.7.0",
     "@tauri-apps/plugin-opener": "^2.5.3",