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/nav.html

@@ -0,0 +1,11 @@
+<nav class="main-nav" data-testid="main-nav" aria-label="Main navigation">
+    <a href="/" class="nav-link {% if active_page == 'home' %}active{% endif %}" data-testid="nav-link-home">
+        Home
+    </a>
+    <a href="/posts" class="nav-link {% if active_page == 'posts' %}active{% endif %}" data-testid="nav-link-posts">
+        Posts
+    </a>
+    <a href="/about" class="nav-link {% if active_page == 'about' %}active{% endif %}" data-testid="nav-link-about">
+        About
+    </a>
+</nav>