fix(Docker): ship prebuilt frontend/dist, drop npm from image (removes 'vite not found')

The Vue SPA is now built locally and its frontend/dist/ committed (force-added).
Dockerfile is a single-stage runtime: copy prebuilt dist + python:3.11 + Flask.
No node/npm inside the image -> deterministic, no npm/esbuild postinstall flakiness.
To update UI: edit src, npm run build, commit frontend/dist/.

.dockerignore

@@ -3,7 +3,6 @@ venv/
 **/__pycache__/
 *.pyc
 **/node_modules/
-frontend/dist/
 backend/data/
 data/
 .env