feat: make sharedinbox.de heading a link and add git commit row to about table (#199)

- Wrap the '## sharedinbox.de' heading in a markdown hyperlink to https://sharedinbox.de
- Add a dedicated 'Git Commit' table row with a clickable link to the commit on Codeberg when GIT_HASH is set
- Update clipboard test to assert the heading link is present in copied markdown

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/ui/screens/about_screen.dart

@@ -47,10 +47,14 @@ class _AboutScreenState extends ConsumerState<AboutScreen> {
     final osName = _capitalize(Platform.operatingSystem);
     final isDark = MediaQuery.of(context).platformBrightness == Brightness.dark;
 
-    return '## sharedinbox.de\n\n'
+    final gitCommitLine = _gitHash.isNotEmpty
+        ? '| Git Commit | [$_gitHash](https://codeberg.org/guettli/sharedinbox/commit/$_gitHash) |\n'
+        : '';
+    return '## [sharedinbox.de](https://sharedinbox.de)\n\n'
         '| Property | Value |\n'
         '|----------|-------|\n'
         '| App Version | $versionDisplay |\n'
+        '$gitCommitLine'
         '| Platform | ${Platform.operatingSystem} |\n'
         '| $osName Version | ${Platform.operatingSystemVersion} |\n'
         '| Resolution | ${physW}x$physH px'

test/widget/about_screen_test.dart

@@ -151,6 +151,10 @@ void main() {
     expect(clipboardText, contains('Dark Mode'));
     expect(clipboardText, contains('IMAP Accounts'));
     expect(clipboardText, contains('JMAP Accounts'));
+    expect(
+      clipboardText,
+      contains('[sharedinbox.de](https://sharedinbox.de)'),
+    );
   });
 
   testWidgets('AboutScreen create-issue button opens Codeberg URL', (