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

.gitea/workflows/check.yml

@@ -28,10 +28,10 @@ jobs:
 
       - name: Fingerprint the build context
         # Writes the hash of the commit being checked into the context, which
-        # invalidates the `COPY . .` layer of both check stages: a commit
-        # that was never linted, format-checked, tested and built cannot
-        # report success from cache.
+        # invalidates the `COPY . .` layer of every check stage: a commit
+        # that was never linted, format-checked, stylesheet-checked, tested
+        # and built cannot report success from cache.
         run: git rev-parse HEAD > .ci-fingerprint
 
-      - name: Build Docker image (runs make fmt-check, golangci-lint, make test, make build)
+      - name: Build Docker image (runs make fmt-check, golangci-lint, the stylesheet check, make test, make build)
         run: script/cibuild

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; \
     }

README.md

@@ -1398,13 +1398,13 @@ the `dist.integrity` hash listed at
 ## Stylesheet
 
 `static/css/tailwind.css` is generated by Tailwind and committed. To change the
-styles, edit the templates, `static/js/app.js` or `static/css/input.css`, run
-`make css`, and commit the regenerated file with the change. Tailwind takes
-classes only from the files that `input.css` names in its `@source` lines; a
-class written in any other file is not generated until that file is named there
-too. `make check` and the image build fail when the committed file differs from
-what `make css` generates. `static/css/style.css` is hand-written and is not
-generated.
+styles, edit the templates, `static/js/app.js`,
+`internal/handlers/recent_events.go` or `static/css/input.css`, run `make css`,
+and commit the regenerated file with the change. Tailwind takes classes only
+from the files that `input.css` names in its `@source` lines; a class written in
+any other file is not generated until that file is named there too. `make check`
+and the image build fail when the committed file differs from what `make css`
+generates. `static/css/style.css` is hand-written and is not generated.
 
 `make css` runs the Tailwind standalone CLI in Docker, at the version and sha256
 pinned in the Dockerfile's stylesheet stages; it is never installed on the host.