fix: Bugs created after the overhaul, performance and layout fixes

backend/Dockerfile

@@ -1,6 +1,6 @@
 FROM python:3.11-slim
 
-# WeasyPrint system dependencies (libpango, libcairo, etc.)
+# System dependencies: WeasyPrint (pango/cairo), ffmpeg (video thumbs), poppler (pdf2image)
 RUN apt-get update && apt-get install -y --no-install-recommends \
     libpango-1.0-0 \
     libpangocairo-1.0-0 \
@@ -8,6 +8,8 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     libffi-dev \
     shared-mime-info \
     fonts-dejavu-core \
+    ffmpeg \
+    poppler-utils \
     && apt-get clean && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app