Instant VM/LXC modals via bulk hydration + server prewarmer

Backend keeps every guest's modal payload (details, backups, apps, schedule, mount points) warm in-memory and exposes them through a single `/api/vms/modal-cache-all` endpoint. The dashboard hydrates its entire modal cache from that one request on page load, so opening any guest — first click or after coming back later — renders instantly. Replaces ~84 per-guest fetches with 1.

AppImage/scripts/notification_events.py

@@ -3516,6 +3516,17 @@ class PollingCollector:
         try:
             import lxc_apps
             lxc_apps.refresh_all_apps(force=False)
+            # After the refresh, emit `app_update_available` for every
+            # sidecar entry currently flagged with a pending upstream
+            # release. `check_app(force=False)` short-circuits on a
+            # fresh `checked_at` and never reaches the emit path, so
+            # without this call the notification only ever fired on
+            # the exact tick where a new version was FIRST observed —
+            # missed forever if the user had the toggle off at that
+            # moment. `notification_manager` dedups by entity_id
+            # (vmid + app_id + latest_version) so repeated calls only
+            # deliver one notification per release.
+            lxc_apps.emit_all_pending_updates()
         except Exception as e:
             print(f"[PollingCollector] lxc_apps refresh failed: {e}")