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/templates/partials/footer.html

@@ -0,0 +1,14 @@
+<footer class="site-footer" data-testid="site-footer">
+    <div class="container" data-testid="footer-container">
+        <div class="footer-copyright" data-testid="footer-copyright">
+            <span data-testid="copyright-text">© 2026 Blog. All rights reserved.</span>
+        </div>
+        
+        <nav class="footer-links" data-testid="footer-nav" aria-label="Footer navigation">
+            <a href="/about" class="footer-link" data-testid="footer-link-about">About</a>
+            <a href="/privacy" class="footer-link" data-testid="footer-link-privacy">Privacy</a>
+            <a href="/terms" class="footer-link" data-testid="footer-link-terms">Terms</a>
+            <a href="/api/docs" class="footer-link" data-testid="footer-link-api">API</a>
+        </nav>
+    </div>
+</footer>