fix(docker): add librsvg2-bin to Linux container for rsvg-convert

Quarto's Lua filter calls rsvg-convert to convert SVG figures to PDF
during PDF builds. librsvg2-dev was present (C headers/lib) but the
binary package librsvg2-bin was missing, causing a FATAL build error:
  'Could not convert a SVG to a PDF. Please ensure rsvg-convert is on path'

Also adds rsvg-convert to the Phase 2 verification checks so missing
tools are caught at image build time, not at render time.

book/docker/linux/Dockerfile

@@ -82,6 +82,7 @@ RUN echo "🚀 === STARTING SYSTEM DEPENDENCIES INSTALLATION ===" && \
     echo "  - libharfbuzz-dev" && \
     echo "  - libfribidi-dev" && \
     echo "  - librsvg2-dev" && \
+    echo "  - librsvg2-bin" && \
     echo "  - libgdal-dev" && \
     echo "  - libudunits2-dev" && \
     echo "  - wget" && \
@@ -107,6 +108,7 @@ RUN echo "🚀 === STARTING SYSTEM DEPENDENCIES INSTALLATION ===" && \
     libharfbuzz-dev \
     libfribidi-dev \
     librsvg2-dev \
+    librsvg2-bin \
     libgdal-dev \
     libudunits2-dev \
     wget \
@@ -133,6 +135,12 @@ RUN echo "🚀 === STARTING SYSTEM DEPENDENCIES INSTALLATION ===" && \
         echo "❌ git not found"; \
         exit 1; \
     fi && \
+    if command -v rsvg-convert >/dev/null 2>&1; then \
+        echo "📦 rsvg-convert available"; \
+    else \
+        echo "❌ rsvg-convert not found"; \
+        exit 1; \
+    fi && \
     \
     echo "🧹 Cleaning package cache..." && \
     rm -rf /var/lib/apt/lists/* && \