Improve help modal navigation with keyboard and search support

Refactor help navigation to support keyboard arrow keys, content pane
focus
toggling, and enhanced search filtering that hides non-matching
sections.
Improve mobile toggle behavior and accessibility with proper focus
management.

frontend/index.html

@@ -774,7 +774,7 @@
           <div class="help-nav-header">
             <div class="help-nav-title">Sommaire</div>
             <button class="help-nav-toggle" id="help-nav-toggle" title="Afficher/Masquer le sommaire" aria-label="Menu sommaire">
-              <i data-lucide="menu" style="width:16px;height:16px"></i>
+              <i data-lucide="chevron-down" style="width:16px;height:16px"></i>
             </button>
           </div>
           <div class="help-nav-search-wrap">