chore: add a recipe for local act runs

Assisted-by: Delta:zai-glm-5.3

Justfile

@@ -21,3 +21,7 @@ watch: tools
 # Remove generated output and build caches
 clean:
     ./scripts/clean.sh
+
+# Run the CI workflow locally with act (requires act + docker)
+ci:
+    act push -W .gitea/workflows/build.yml -j build --artifact-server-path /tmp/act-artifacts

README.md

@@ -24,6 +24,7 @@ checksum-verified into `tools/` on first use (no global installs, no cargo):
     just preview  # build, then serve the site locally
     just watch    # rebuild on change and serve
     just clean    # remove public/ and build caches
+    just ci       # run the CI workflow locally with act (needs act + docker)
 
 ## Adding a page