Install graphviz where doxygen runs

The submodule fix got the cmake_build job through checkout, dependencies,
configure and the whole compile for the first time in the run history -- and
into the next latent failure: `doxygen Doxyfile` generates dot graphs and the
Doxyfile fails on warnings, but the runner image has no `dot`, so every graph
reported "Problems running dot: exit code=127" and the step failed after an
otherwise clean build. Locally graphviz was installed, which is why the
pre-flight missed it. One package in the one job that builds documentation.

Co-Authored-By: Claude Code (Claude Fable 5, claude-fable-5) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KzBDV2fqgnUAcqCKqKvc71

.gitea/workflows/ci.yaml

@@ -14,7 +14,7 @@ jobs:
         run: |
           sudo apt-get update -y
           sudo apt-get install -y \
-            cmake doxygen gcc gcovr pkg-config \
+            cmake doxygen gcc gcovr graphviz pkg-config \
             libasound2-dev libfreetype-dev libharfbuzz-dev \
             libpng-dev libtiff-dev libwebp-dev \
             libudev-dev libx11-dev libxcursor-dev libxext-dev \