feat(nav): header unifié desktop/mobile (issue #16)

Partiel _header.html unique pour les 19 pages (fin des copies qui
dérivaient): desktop une ligne — primaires + menu Plus + Ctrl K,
page active déduite du gabarit dans render(); mobile ≤900px
burger + panneau ≥44px, Échap/clic extérieur; wallboard sans
header et triage sur le même partiel. nav.js partagé.

app/templates/board.html

@@ -12,30 +12,7 @@
     <script src="/static/js/board.js" defer></script>
 </head>
 <body>
-    <header class="app-header">
-        <div class="brand">
-            <a href="/" class="logo" aria-label="Kanbante — accueil"><span class="logo-mark">📋</span><span class="gradient-text">Kanbante</span></a>
-            <span class="crumb"><span class="sep">/</span> {{ owner }} / <strong>{{ repo }}</strong></span>
-        </div>
-        <nav class="nav-pills">
-            <a href="/" class="nav-pill">🏠 Projets</a>
-            <a href="/focus" class="nav-pill">🎯 Focus</a>
-            <a href="/today" class="nav-pill">🌅 Aujourd'hui</a>
-            <a href="/my" class="nav-pill">👤 Mes issues</a>
-            <a href="/activity" class="nav-pill">📰 Activité</a>
-            <a href="/search" class="nav-pill">🔍 Recherche</a>
-            <a href="/capture" class="nav-pill">⚡ Capture</a>
-            <a href="/digest" class="nav-pill">🧠 Digest</a>
-            <a href="/forecast" class="nav-pill">🔮 Prévisions</a>
-            <a href="/rules" class="nav-pill">⚙️ Règles</a>
-            <a href="/health" class="nav-pill">🩺 Santé</a>
-            <a href="/wall" class="nav-pill">🖥️ Wallboard</a>
-            <button type="button" class="nav-pill palette-trigger" title="Palette de commandes (Ctrl+K)">⌨️ Ctrl K</button>
-            {% if repo_info.html_url %}
-            <a href="{{ repo_info.html_url }}" target="_blank" class="nav-pill">🔗 Gitea ↗</a>
-            {% endif %}
-        </nav>
-    </header>
+    {% include "_header.html" %}
 
     {% if milestones %}
     <div class="milestone-bar">