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.playwright

@@ -0,0 +1,21 @@
+FROM mcr.microsoft.com/playwright:v1.58.0-noble
+
+WORKDIR /app
+
+RUN apt-get update && apt-get install -y unzip \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN curl -fsSL https://bun.sh/install | bash
+ENV PATH="/root/.bun/bin:$PATH"
+
+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