fix: resolve 3 critical UI bugs

1. Refresh button no longer doubles header
   - Added ?fragment=1 param to filter/refresh HTMX requests
   - Board route returns board_fragment.html (columns only) for fragment requests
   - Refresh button uses window.location.reload() for full page reload

2. No more raw JSON in header
   - base.html: user info rendered server-side via template variable
   - board.py: decodes session cookie, passes user to template
   - Dashboard/board templates show Login/Deconnexion based on auth state

3. New issue form works
   - Replaced HTMX post with JS fetch (handleNewIssue)
   - Adds CSRF token to form submission
   - Reloads page on successful creation
   - Cleaner error handling

app/templates/base.html

@@ -18,9 +18,17 @@
       </a>
     </div>
     <div class="header-right">
-      <span id="user-info" hx-get="/auth/user" hx-trigger="load" hx-swap="innerHTML">
+      {% if user %}
+      <span id="user-info">
+        {% if user.avatar_url %}<img src="{{ user.avatar_url }}" width="20" height="20" class="avatar">{% endif %}
+        {{ user.login }}
+        <a href="/auth/logout" class="btn btn-sm">Déconnexion</a>
+      </span>
+      {% else %}
+      <span id="user-info">
         <a href="/auth/login" class="btn btn-sm">🔑 Login</a>
       </span>
+      {% endif %}
       <button class="theme-toggle" onclick="toggleTheme()" title="Thème clair/sombre">🌓</button>
     </div>
   </header>
@@ -28,8 +36,7 @@
     {% block content %}{% endblock %}
   </main>
 
-  <!-- Card Detail Modal (global) -->
-  <div id="card-modal" class="modal-overlay" style="display:none" hx-target="#card-modal-content">
+  <div id="card-modal" class="modal-overlay" style="display:none">
     <div class="modal-container">
       <div class="modal-header">
         <h3 id="card-modal-title">Détail</h3>
@@ -40,5 +47,6 @@
   </div>
 
   <script src="/static/js/app.js" defer></script>
+  {% block scripts %}{% endblock %}
 </body>
 </html>