feat: M4 (part 2) — React reader UI with theming

- Vite + React + TS + Tailwind SPA served by FastAPI (multi-stage Docker build)
- Four color schemes (Midnight default, Forest, Slate, YouTube) x dark/light,
  adjustable text size; persisted to user preferences and localStorage
- Header search, grid/list toggle, theme menu; sidebar filters (show state,
  sort, include Shorts/live, language + topic tag chips with any/all matching)
- Infinite-scroll feed of video cards; click opens youtube.com in a new tab;
  per-video watched / saved / hidden actions with optimistic updates
- SPA fallback routing; login screen for unauthenticated users

Dockerfile

@@ -0,0 +1,28 @@
+# Stage 1: build the frontend SPA
+FROM node:20-alpine AS frontend
+WORKDIR /fe
+COPY frontend/package.json ./
+RUN npm install
+COPY frontend/ ./
+RUN npm run build
+
+# Stage 2: backend runtime, serving the built SPA
+FROM python:3.12-slim
+ENV PYTHONDONTWRITEBYTECODE=1 \
+    PYTHONUNBUFFERED=1 \
+    PIP_NO_CACHE_DIR=1 \
+    PIP_DISABLE_PIP_VERSION_CHECK=1
+
+WORKDIR /app
+
+COPY backend/requirements.txt .
+RUN pip install -r requirements.txt
+
+COPY backend/ .
+COPY --from=frontend /fe/dist ./app/static_spa
+
+RUN adduser --disabled-password --gecos "" appuser && chown -R appuser /app
+USER appuser
+
+EXPOSE 8000
+CMD ["sh", "entrypoint.sh"]