Remove yarn, use npm exclusively

Replace all yarn references with npm across README, CLAUDE.md,
CHANGELOG, .gitignore, and CI workflow. Remove yarn.lock.

Co-Authored-By: Claude <noreply@anthropic.com>

.gitea/workflows/release.yml

@@ -113,7 +113,6 @@ jobs:
             -x "${THEME_NAME}/src/*" \
             -x "${THEME_NAME}/package.json" \
             -x "${THEME_NAME}/package-lock.json" \
-            -x "${THEME_NAME}/yarn.lock" \
             -x "${THEME_NAME}/composer.json" \
             -x "${THEME_NAME}/composer.lock" \
             -x "${THEME_NAME}/.gitignore" \