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/.gitignore

@@ -18,6 +18,8 @@
 /public/storage
 /storage/*.key
 /storage/pail
+/public/downloads/*.apk
+/public/downloads/*.ipa
 /vendor
 Homestead.json
 Homestead.yaml