feat: page mises à jour avec liste des paquets par cible

- Backend: GET /api/updates/targets (pct list via SSH)
- Backend: GET /api/updates/packages?target= (apt list --upgradable)
- Frontend: grille de cards par cible (host + chaque LXC)
- Bouton Check/Update par card, liste paquets dépliable (version actuelle → nouvelle)
- Boutons globaux "Tout vérifier" et "Tout mettre à jour"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/locales/en.json

@@ -78,7 +78,7 @@
     "disconnected": "Disconnected"
   },
   "updates": {
-    "desc": "Run apt updates on the host or LXC containers.",
+    "desc": "Check and run apt updates on the host or LXC containers.",
     "selectTarget": "Select target",
     "targetHost": "Proxmox Host",
     "targetAll": "All LXC",
@@ -87,6 +87,16 @@
     "output": "Output",
     "history": "History",
     "noHistory": "No updates performed",
+    "checkUpdates": "Check",
+    "checkAll": "Check all",
+    "updateTarget": "Update",
+    "updateAll": "Update all",
+    "packagesToUpdate": "package(s) to update",
+    "upToDate": "Up to date",
+    "notChecked": "Not checked",
+    "checking": "Checking...",
+    "loadingTargets": "Loading targets...",
+    "stopped": "Stopped",
     "status": {
       "running": "Running",
       "success": "Success",