feat(links): surface source repo + Weblate links on site and in app

- site: set sourceURL (git.comunes.org/comunes/tane) and weblateURL
  (translate.comunes.org/projects/tane) — activates footer + collaborate links
- app About screen: add 'Source code' and 'Help translate' link tiles
- i18n: add about.sourceCode / about.translate / about.translateSubtitle
  (en base + es/pt/fr/de/ast; ja falls back to base), regenerate slang
- docs: add TRANSLATIONS.md runbook for the Weblate app component + git
  token + slang-regeneration workflow

apps/app_seeds/lib/ui/about_screen.dart

@@ -9,6 +9,12 @@ import 'theme.dart';
 /// The app's public website. Shown as a tappable row in [AboutScreen].
 const String _websiteUrl = 'https://tane.comunes.org';
 
+/// The public source repository (AGPL). Shown as a tappable row in [AboutScreen].
+const String _sourceUrl = 'https://git.comunes.org/comunes/tane';
+
+/// The translation platform where anyone can help translate Tane.
+const String _translateUrl = 'https://translate.comunes.org/projects/tane/';
+
 /// The year Tane's copyright starts. Ğ1nkgo uses its own first year (2023);
 /// Tane's is 2026.
 const int _copyrightStartYear = 2026;
@@ -69,6 +75,24 @@ class AboutScreen extends StatelessWidget {
               mode: LaunchMode.externalApplication,
             ),
           ),
+          ListTile(
+            leading: const Icon(Icons.code, color: seedGreen),
+            title: Text(t.about.sourceCode),
+            subtitle: const Text(_sourceUrl),
+            onTap: () => launchUrl(
+              Uri.parse(_sourceUrl),
+              mode: LaunchMode.externalApplication,
+            ),
+          ),
+          ListTile(
+            leading: const Icon(Icons.translate, color: seedGreen),
+            title: Text(t.about.translate),
+            subtitle: Text(t.about.translateSubtitle),
+            onTap: () => launchUrl(
+              Uri.parse(_translateUrl),
+              mode: LaunchMode.externalApplication,
+            ),
+          ),
           ListTile(
             leading: const Icon(Icons.privacy_tip_outlined, color: seedGreen),
             title: Text(t.legal.title),