feat(vendor): replace CDN dependencies with local npm packages

Introduces a vendor build pipeline so all third-party browser assets
(Bootstrap, Bootstrap Icons, Font Awesome, marked, jQuery) are served
from local static files instead of external CDNs.

- Add app/package.json with vendor deps and postinstall/build scripts
- Add app/scripts/copy-vendor.js to copy assets to static/vendor/
- Update base.html.j2 to use url_for('static', ...) for all vendor assets
- Update Dockerfile to install Node.js/npm and run npm install
- Update .gitignore to exclude app/node_modules/ and app/static/vendor/

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/package.json

@@ -1,5 +1,16 @@
 {
+  "dependencies": {
+    "@fortawesome/fontawesome-free": "^6.7.2",
+    "bootstrap": "5.2.2",
+    "bootstrap-icons": "1.9.1",
+    "jquery": "3.6.0",
+    "marked": "^4.3.0"
+  },
   "devDependencies": {
     "cypress": "^14.5.1"
+  },
+  "scripts": {
+    "build": "node scripts/copy-vendor.js",
+    "postinstall": "node scripts/copy-vendor.js"
   }
 }