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/.

frontend/dist/assets/_plugin-vue_export-helper-DlAUqK2U.js

@@ -0,0 +1 @@
+const s=(t,r)=>{const o=t.__vccOpts||t;for(const[c,e]of r)o[c]=e;return o};export{s as _};