feat: remove legacy frontend, root routing, compliance ZIP with Chromium PDF, chat markdown styling

- Legacy frontend removed: React SPA served at / (no /app/ prefix)
- Compliance report ZIP: Chromium headless --print-to-pdf (identical to browser) + CSV findings
- Server-side generation state: .compliance_generating marker survives page navigation
- RAG enriched: Description + Rationale + Remediation + Audit extracted from CIS vector chunks
- Chat markdown: syntax highlighting (Catppuccin), styled headings/lists/tables/blockquotes/code
- Default model auto-selected on Chat Agent load

backend/Dockerfile

@@ -4,7 +4,8 @@ WORKDIR /app
 
 # Install system deps + Terraform CLI
 RUN apt-get update && apt-get install -y --no-install-recommends \
-    curl gcc libffi-dev unzip && \
+    curl gcc libffi-dev unzip \
+    chromium && \
     ARCH=$(dpkg --print-architecture) && \
     curl -fsSL "https://releases.hashicorp.com/terraform/1.7.5/terraform_1.7.5_linux_${ARCH}.zip" -o /tmp/tf.zip && \
     unzip /tmp/tf.zip -d /usr/local/bin/ && rm /tmp/tf.zip && \