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

pyproject.toml

@@ -14,6 +14,7 @@ dependencies = [
     "asyncpg>=0.30.0",
     "dishka>=1.5.0",
     "httpx>=0.28.0",
+    "jinja2>=3.1.6",
 ]
 
 [build-system]