docs: add Phase 0 documentation

- docs/ARCHITECTURE.md - Clean Architecture patterns and conventions
- docs/API_INTEGRATION.md - Core API and Tatlock API endpoints
- docs/DEPLOYMENT.md - Docker, NPM, Portainer setup
- docs/DATAGRID.md - DataGrid component specification
- docs/THEMING.md - Material 3 theming guide

Also update seed color to teal for consistency.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/main.dart

@@ -23,7 +23,7 @@ class TatlockApp extends StatelessWidget {
     return MaterialApp(
       title: 'Tatlock UI',
       theme: ThemeData(
-        colorScheme: ColorScheme.fromSeed(seedColor: Colors.indigo),
+        colorScheme: ColorScheme.fromSeed(seedColor: Colors.teal),
         useMaterial3: true,
       ),
       home: const HomePage(),