docs: add Gitea version compatibility matrix and badge

- Create COMPATIBILITY.md with version matrix, Gitea release tracking,
  template variable reference, and compatibility verification process
- Add Gitea 1.25+ | 1.26.4 tested badge to README
- Update all 6 READMEs (EN + 5 translations) with latest tested version
  and link to COMPATIBILITY.md
- Include COMPATIBILITY.md in release artifacts

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

.github/workflows/release.yml

@@ -33,7 +33,7 @@ jobs:
           VERSION=${GITHUB_REF#refs/tags/}
           ARCHIVE="gitea-mail-templates-${VERSION}"
           mkdir -p dist
-          cp -r themes LICENSE README.md "dist/${ARCHIVE}"
+          cp -r themes LICENSE README.md COMPATIBILITY.md "dist/${ARCHIVE}"
           cd dist
           zip -r "${ARCHIVE}.zip" "${ARCHIVE}"
           tar -czf "${ARCHIVE}.tar.gz" "${ARCHIVE}"