feat(ui): add web UI with Jinja2 templates and Gitea themes

- Add Jinja2 templates with data-testid attributes for testing
- Create light/dark themes based on Gitea color scheme
- Add theme switching with localStorage persistence
- Create base CSS, components, and layout styles
- Add mock web routes for UI demonstration
- Register web router and static files in main.py
- Add data-testid requirements to AGENTS.md
- Install jinja2 dependency

app/presentation/web/__init__.py

@@ -0,0 +1,13 @@
+"""Web UI layer for blog application.
+
+This package provides HTML endpoints and templates for the blog web interface,
+separate from the JSON API layer. Uses Jinja2 templates with Gitea-inspired
+theme support and comprehensive data-testid attributes for testing.
+
+The web layer follows the same DDD principles as the API layer and will
+be integrated with use cases in future iterations.
+"""
+
+from app.presentation.web.routes import router
+
+__all__ = ["router"]