Vendor backend and frontend into the monorepo

The submodule collapse was only half applied: .gitmodules was deleted but
backend/ and frontend/ were still recorded as gitlinks, so none of their
files were tracked. Replace the gitlinks with the real trees.

Also untrack .env (it carried placeholder secrets) in favour of a tracked
.env.example, drop the committed __pycache__, and narrow the blanket *.png
ignore that would have swallowed design assets.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/Dockerfile

@@ -0,0 +1,26 @@
+# Stage 0, "build-stage", based on Bun, to build and compile the frontend
+FROM oven/bun:1 AS build-stage
+
+WORKDIR /app
+
+COPY package.json bun.lock /app/
+
+COPY frontend/package.json /app/frontend/
+
+WORKDIR /app/frontend
+
+RUN bun install
+
+COPY ./frontend /app/frontend
+ARG VITE_API_URL
+
+RUN bun run build
+
+
+# Stage 1, based on Nginx, to have only the compiled app, ready for production with Nginx
+FROM nginx:1
+
+COPY --from=build-stage /app/frontend/dist/ /usr/share/nginx/html
+
+COPY ./frontend/nginx.conf /etc/nginx/conf.d/default.conf
+COPY ./frontend/nginx-backend-not-found.conf /etc/nginx/extra-conf.d/backend-not-found.conf