style(aurora): CSS class-based refactor, full i18n, and locale alignment

themes/aurora/ (all 11 tmpl templates):
- Extract all reusable inline styles to CSS classes in <style> blocks
- Organize CSS with /* Section */ comment headers (Base, Header, Card, etc.)
- Clean HTML structure: <tr>/<td> on separate lines, consistent 2-space indent
- Replace all hardcoded English button text with locale.Tr keys
- Add mail.team_invite.text_3 disclaimer notice to team_invite
- Go variable assignments on dedicated lines with proper indentation
- Section comments in Title Case

tools/preview/locale.go:
- Replace LocaleEN with full 60-key official Gitea mail locale
- Add positional format specifier support (%[1]s, %[2]d) in Tr()
- All keys verified against official Gitea locale_en-US.json (100% match)

.gitignore:
- Add preview/rendered.js entry

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -20,3 +20,6 @@ Desktop.ini
 
 # Package lock file
 package-lock.json
+
+# Preview generated files
+preview/rendered.js