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/templates/settings/plugins.html

@@ -62,6 +62,20 @@
       </label>
     </div>
 
+    {# Failure notice — shown when this plugin failed to load #}
+    {% set fail_reason = plugin_failures.get(plugin._dir) %}
+    {% if fail_reason %}
+    <div style="display:flex;align-items:flex-start;gap:0.5rem;padding:0.5rem 0.75rem;
+                margin-top:0.6rem;background:color-mix(in srgb,var(--red) 10%,var(--bg));
+                border:1px solid color-mix(in srgb,var(--red) 30%,var(--border));
+                border-radius:5px;font-size:0.82rem;">
+      <span style="color:var(--red);flex-shrink:0;">✕</span>
+      <span><strong style="color:var(--red);">Failed to load</strong>
+        <span style="color:var(--text-muted);margin-left:0.3rem;">— {{ fail_reason }}</span>
+      </span>
+    </div>
+    {% endif %}
+
     {# Config fields — only shown when there are config keys #}
     {% if plugin.get('config') %}
     <div style="border-top:1px solid var(--border);padding-top:1rem;display:grid;gap:0.6rem;">