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/app.py

@@ -122,7 +122,13 @@ def create_app(
         from .core.plugin_manager import load_plugins
         load_plugins(app)
 
-    # ── 10. Share-token middleware ─────────────────────────────────────────────
+    # ── 10. Template context: inject plugin_failures into every response ───────
+    @app.context_processor
+    def _inject_plugin_failures():
+        from .core.plugin_manager import get_plugin_failures
+        return {"plugin_failures": get_plugin_failures()}
+
+    # ── 11. Share-token middleware ─────────────────────────────────────────────
     @app.before_request
     async def _validate_share_token():
         """If ?s=<token> is present, validate it and set g.share_viewer."""