Integrate Bootstrap 5 and Bootstrap Icons into UI

- Add Bootstrap 5.3.3 + Icons via CDN to base.html
- Replace 315-line hand-written CSS with 35-line brand overrides
- Update all 13 templates with Bootstrap utility classes:
  - Responsive navbar with mobile hamburger menu
  - Consistent card-based layout for forms and profile
  - Proper button alignment with d-flex and d-grid utilities
  - List groups for data display (profile info)
  - Professional alerts and icons
- No backend changes, no build toolchain needed
- Responsive design works on mobile/tablet/desktop

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

web/templates/message.html

@@ -2,11 +2,17 @@
 {% block title %}{{ title }} — EvoSync{% endblock %}
 
 {% block content %}
-<div class="message-card">
-    <h1>{{ title }}</h1>
-    <p>{{ message }}</p>
-    {% if link %}
-        <p><a href="{{ link }}">{{ link_text }}</a></p>
-    {% endif %}
+<div class="row justify-content-center">
+    <div class="col-sm-10 col-md-6 col-lg-5">
+        <div class="card shadow-sm mt-5 text-center">
+            <div class="card-body p-5">
+                <h1 class="h4 mb-3">{{ title }}</h1>
+                <p class="text-muted">{{ message }}</p>
+                {% if link %}
+                    <a href="{{ link }}" class="btn btn-primary mt-2">{{ link_text }}</a>
+                {% endif %}
+            </div>
+        </div>
+    </div>
 </div>
 {% endblock %}