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/base.html

@@ -243,6 +243,20 @@ function setTimeRange(val) {
 </script>
 
     <main>
+        {% if plugin_failures and session.user_role == 'admin' %}
+        <div style="background:color-mix(in srgb,var(--yellow) 12%,var(--bg-elevated));
+                    border:1px solid color-mix(in srgb,var(--yellow) 35%,var(--border));
+                    border-radius:6px;padding:0.6rem 1rem;margin-bottom:1rem;
+                    font-size:0.84rem;display:flex;align-items:flex-start;gap:0.6rem;">
+            <span style="color:var(--yellow);flex-shrink:0;">⚠</span>
+            <span>
+                <strong style="color:var(--text);">
+                    {{ plugin_failures | length }} plugin{{ 's' if plugin_failures | length != 1 }} failed to load.
+                </strong>
+                <a href="/settings/plugins/" style="color:var(--accent);margin-left:0.4rem;">View details →</a>
+            </span>
+        </div>
+        {% endif %}
         {% if error %}
         <div class="alert alert-error">{{ error }}</div>
         {% endif %}