Show the rules a stale stylesheet differs in (closes #231)

The css-check stage splits both stylesheets after each "}" and prints
their diff, so a failure names the rules that differ instead of a
character position.

The check workflow's fingerprint comment and build step name now
include the stylesheet check, as the README does. The README's
Stylesheet section names internal/handlers/recent_events.go among the
files to edit.

Model: opus-5-5

Dockerfile

@@ -55,8 +55,12 @@ RUN tailwindcss -i static/css/input.css -o /out/tailwind.css --minify
 FROM scratch AS css-output
 COPY --from=css /out/tailwind.css /
 
+# Both files are split after each "}", one rule per line, so that when they
+# differ the diff shows the rules that differ.
 FROM css AS css-check
-RUN cmp /out/tailwind.css static/css/tailwind.css || { \
+RUN sed 's/}/}\n/g' static/css/tailwind.css > /tmp/committed.css \
+    && sed 's/}/}\n/g' /out/tailwind.css > /tmp/generated.css \
+    && diff -U0 /tmp/committed.css /tmp/generated.css || { \
         echo "static/css/tailwind.css is not what make css generates; run make css" >&2; \
         exit 1; \
     }