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