feat: add comprehensive loading feedback (#18)

frontend/src/app/app.html

@@ -1,11 +1,23 @@
-@if (!session.isLoggedIn()) {
+@if (session.status() === 'checking') {
+  <section class="session-state" role="status" aria-live="polite">
+    <app-brand-mark class="brand-mark" />
+    <mat-spinner diameter="34" />
+    <strong>Vérification de votre session…</strong>
+  </section>
+} @else if (session.status() === 'error') {
+  <section class="session-state" role="alert">
+    <app-brand-mark class="brand-mark" />
+    <strong>Impossible de vérifier votre session</strong>
+    <p>Vérifiez votre connexion puis réessayez.</p>
+    <button mat-flat-button type="button" class="primary-action" (click)="session.refresh()">
+      Réessayer
+    </button>
+  </section>
+} @else if (!session.isLoggedIn()) {
   <app-sign-in-page (loginRequested)="login()" />
 } @else {
   <div class="workspace-shell">
-    <app-workspace-navigation
-      [userInitial]="userInitial()"
-      (logoutRequested)="logout()"
-    />
+    <app-workspace-navigation [userInitial]="userInitial()" (logoutRequested)="logout()" />
 
     <main class="workspace-main">
       <header class="mobile-header">