feat: plugin fault isolation notices + NUT managed mode fixes

Plugin failures:
- Track load-time failures in _FAILED_PLUGINS dict with human-readable reasons
- Inject plugin_failures into all templates via context processor
- Show admin-only warning banner in base.html when any plugin fails to load
- Show inline failure notice per plugin card in Settings → Plugins

NUT managed mode:
- entrypoint.sh: make nut_setup.py failure non-fatal so the container starts
  even if nut_ups_host isn't set yet (allows fixing config via UI)
- save_plugins: when nut_managed is enabled, auto-set nut_host=localhost so
  the plugin connects to the managed NUT instance without extra manual config

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

fabledscryer/settings/routes.py

@@ -408,6 +408,12 @@ async def save_plugins():
                                 plugin_cfg[cfg_key] = default_val
                         else:
                             plugin_cfg[cfg_key] = raw_val
+                # Managed NUT: when nut_managed is on, the NUT daemon runs in this
+                # container — force nut_host/port to localhost so the plugin
+                # connects to it automatically without manual config.
+                if name == "ups" and plugin_cfg.get("nut_managed"):
+                    plugin_cfg["nut_host"] = "localhost"
+                    plugin_cfg["nut_port"] = 3493
                 await set_setting(db, f"plugin.{name}", plugin_cfg)
     await _reload_app_config()
     from fabledscryer.core.plugin_index import clear_catalog_cache