fix: update push commit data paths for Gitea 1.27.1 compatibility

Gitea 1.27.1 fixed the mail template for push events (#38467) —
commit data must now be accessed via .UserCommit.GitCommit instead
of directly on the commit object.

Changed across all 10 themes:
- {{.ID}}               → {{.UserCommit.GitCommit.ID}}
- {{.ID.String}}        → {{.UserCommit.GitCommit.ID.String}}
- {{.MessageTitle}}     → {{.UserCommit.GitCommit.MessageTitle}}

Verification:
- 3 mail-related files changed in Gitea v1.27.0→v1.27.1
- All 110 templates (10 themes × 11 types) render successfully
- No breaking changes — data structure unchanged, only access path

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

README.md

@@ -2,7 +2,7 @@
 
 A curated collection of professionally designed, audience-driven email templates for self-hosted [Gitea](https://about.gitea.com) instances.
 
-[![Gitea](https://img.shields.io/badge/Gitea-1.25+%20%7C%201.27.1%20pending-yellow)](COMPATIBILITY.md) <!-- TRACKER:BADGE -->
+[![Gitea](https://img.shields.io/badge/Gitea-1.25+%20%7C%201.27.1%20tested-blue)](COMPATIBILITY.md) <!-- TRACKER:BADGE -->
 
 > **110 template files — 10 visual styles, 11 email types each**