feat: page de telechargement des applications mobiles (Android / iOS)

Page publique /telecharger reprenant le modele de Ressources-relationnel :
- config/mobile.php (lien de store prioritaire, sinon binaire local, sinon 'bientot')
- DownloadController + vue au style DSFR, lien dans la sidebar
- variables MOBILE_* et depot public/downloads/ (binaires gitignores)
- 3 tests (page publique, lien store, bouton desactive)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

WEB/tests/Feature/DownloadTest.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace Tests\Feature;
+
+use PHPUnit\Framework\Attributes\Test;
+use Tests\TestCase;
+
+/**
+ * Page publique de téléchargement des applications mobiles.
+ */
+class DownloadTest extends TestCase
+{
+    #[Test]
+    public function la_page_de_telechargement_est_publique()
+    {
+        $this->get('/telecharger')
+            ->assertOk()
+            ->assertSee('Android')
+            ->assertSee('Apple');
+    }
+
+    #[Test]
+    public function un_lien_de_store_configure_est_propose()
+    {
+        config(['mobile.android.url' => 'https://play.google.com/store/apps/details?id=fr.cesizen']);
+
+        $this->get('/telecharger')
+            ->assertOk()
+            ->assertSee('play.google.com');
+    }
+
+    #[Test]
+    public function sans_lien_ni_fichier_le_bouton_est_desactive()
+    {
+        config(['mobile.ios.url' => null, 'mobile.ios.file' => 'downloads/inexistant.ipa']);
+
+        $this->get('/telecharger')
+            ->assertOk()
+            ->assertSee('Bientôt disponible');
+    }
+}