Ship pagerite/themes in the built package

hatchling's only-packages=true drops directories without an __init__.py,
so pagerite/themes never made it into the wheel/sdist (this also means
the banner.svg artwork was never packaged). Force-include it as a build
artifact like frontend-build.

pyproject.toml

@@ -44,7 +44,9 @@ source = "vcs"
 packages = ["pagerite"]
 
 [tool.hatch.build]
-artifacts = ["pagerite/frontend-build"]
+# `only-packages` drops directories without an __init__.py, so the theme
+# files must be force-included as artifacts (like the frontend build).
+artifacts = ["pagerite/frontend-build", "pagerite/themes"]
 only-packages = true
 
 [tool.hatch.build.targets.sdist.hooks.custom]