feat: add "Derniers fichiers ouverts" widget to main dashboard

- Replace welcome message with a professional dashboard widget displaying the last opened files.
- Implement configurable file limit via existing settings.
- Design includes responsive grid layout and various UI states (loading, empty, error).
- Introduce new JavaScript module `DashboardRecentWidget` for handling file loading and rendering.
- Ensure accessibility compliance and responsive design breakpoints.

frontend/index.html

@@ -350,10 +350,33 @@
 
       <!-- Content -->
       <main class="content-area" id="content-area" aria-label="Contenu principal">
-        <div class="welcome" id="welcome" role="status">
-          <i data-lucide="library" style="width:48px;height:48px;color:var(--text-muted)"></i>
-          <h2>ObsiGate</h2>
-          <p>Sélectionnez un fichier dans la sidebar ou utilisez la recherche pour commencer.</p>
+        <div id="dashboard-home" class="dashboard-home" role="region" aria-label="Derniers fichiers ouverts">
+          <div class="dashboard-header">
+            <div class="dashboard-title-row">
+              <i data-lucide="clock" class="dashboard-icon"></i>
+              <h2>Derniers fichiers ouverts</h2>
+            </div>
+            <div class="dashboard-filter-row">
+              <select id="dashboard-vault-filter" class="dashboard-select">
+                <option value="">Tous les vaults</option>
+              </select>
+              <span class="dashboard-count" id="dashboard-count"></span>
+            </div>
+          </div>
+          <div id="dashboard-recent-grid" class="dashboard-recent-grid"></div>
+          <div id="dashboard-loading" class="dashboard-loading">
+            <div class="skeleton-card"></div>
+            <div class="skeleton-card"></div>
+            <div class="skeleton-card"></div>
+            <div class="skeleton-card"></div>
+            <div class="skeleton-card"></div>
+            <div class="skeleton-card"></div>
+          </div>
+          <div id="dashboard-recent-empty" class="dashboard-recent-empty hidden">
+            <i data-lucide="inbox"></i>
+            <span>Aucun fichier récent</span>
+            <p>Ouvrez un fichier pour le voir apparaître ici</p>
+          </div>
         </div>
       </main>