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/main.py

@@ -292,6 +292,27 @@ _tpl_env = Environment(
 )
 
 
+# Which main-nav entry a template represents (issue #16): the
+# shared _header.html partial highlights it from this one place
+# instead of 19 hand-maintained copies.
+_NAV_BY_TEMPLATE = {
+    "dashboard.html": "home",
+    "focus.html": "focus",
+    "today.html": "today",
+    "my.html": "my",
+    "activity.html": "activity",
+    "briefing.html": "briefing",
+    "standup.html": "standup",
+    "search.html": "search",
+    "capture.html": "capture",
+    "digest.html": "digest",
+    "forecast.html": "forecast",
+    "rules.html": "rules",
+    "health.html": "health",
+    "health_project.html": "health",
+}
+
+
 def render(
     request: Request, name: str, status_code: int = 200, **context
 ) -> HTMLResponse:
@@ -307,6 +328,7 @@ def render(
         token = secrets.token_urlsafe(32)
     context.setdefault("csrf_token", token)
     context.setdefault("version", APP_VERSION)
+    context.setdefault("active_nav", _NAV_BY_TEMPLATE.get(name, ""))
     html = _tpl_env.get_template(name).render(**context)
     response = HTMLResponse(html, status_code=status_code)
     if generated: