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/pages/post_detail.html

@@ -0,0 +1,58 @@
+{% extends "base.html" %}
+
+{% block title %}{{ post.title }} - Blog{% endblock %}
+{% block meta_description %}{{ post.content.value[:160] }}{% endblock %}
+
+{% block content %}
+<article class="post-detail" data-testid="post-detail">
+    <header class="post-detail-header" data-testid="post-detail-header">
+        <h1 class="post-detail-title" data-testid="post-detail-title">{{ post.title }}</h1>
+        
+        <div class="post-detail-meta" data-testid="post-detail-meta">
+            <span class="post-card-meta-item" data-testid="post-detail-author">
+                <span class="avatar avatar-sm" data-testid="post-detail-author-avatar">{{ post.author_id[0]|upper }}</span>
+                <span data-testid="post-detail-author-name">{{ post.author_id }}</span>
+            </span>
+            <span class="post-card-meta-item" data-testid="post-detail-date">
+                {{ post.created_at.strftime('%B %d, %Y') }}
+            </span>
+            {% if post.published %}
+            <span class="badge badge-success" data-testid="post-detail-status">Published</span>
+            {% else %}
+            <span class="badge" data-testid="post-detail-status">Draft</span>
+            {% endif %}
+        </div>
+    </header>
+    
+    <div class="post-detail-content" data-testid="post-detail-content">
+        {{ post.content.value|nl2br }}
+    </div>
+    
+    <footer class="post-detail-footer" data-testid="post-detail-footer">
+        <div class="post-detail-tags" data-testid="post-detail-tags">
+            {% for tag in post.tags %}
+            <span class="tag" data-testid="post-detail-tag-{{ loop.index }}">{{ tag }}</span>
+            {% endfor %}
+        </div>
+        
+        <div class="divider" data-testid="post-detail-divider"></div>
+        
+        <div class="flex justify-between items-center" data-testid="post-detail-actions">
+            <a href="/" class="btn" data-testid="btn-back-to-posts">
+                ← Back to posts
+            </a>
+            
+            <div class="flex gap-2" data-testid="post-detail-edit-actions">
+                <a href="/posts/{{ post.id }}/edit" class="btn" data-testid="btn-edit-post">
+                    Edit
+                </a>
+                <form action="/posts/{{ post.id }}/delete" method="POST" style="display: inline;" data-testid="form-delete-post">
+                    <button type="submit" class="btn btn-danger" data-testid="btn-delete-post" onclick="return confirm('Are you sure you want to delete this post?');">
+                        Delete
+                    </button>
+                </form>
+            </div>
+        </div>
+    </footer>
+</article>
+{% endblock %}