feat(dashboard): replace 24h charts with Configuration Status and Recent Activity (#785)

* feat(dashboard): replace 24h charts with Configuration Status and Recent Activity

The 24-hour CPU/Memory area charts summed per-container metrics normalized
to each container's CPU quota, producing numbers that bore no honest
relationship to host load. The live ResourceGauges strip already shows
accurate host-level stats, making the historical charts both inaccurate
and redundant.

This commit replaces that row with two side-by-side cards:

- **Configuration Status**: aggregates every toggleable feature on the
  active node (notification agents, alert rules, routing rules, auto-heal,
  auto-update, webhooks, scheduled tasks, MFA, SSO, vulnerability scanning,
  cloud backup, and alert thresholds) into a single at-a-glance card.
  Tier-locked rows display an upgrade indicator instead of a value.
  Each row is clickable and navigates to the relevant settings section.
  Data refreshes every 60 s and immediately on state-invalidate events.

- **Recent Activity**: lists the ten most recent notification-history events
  for the active node (deployments, image updates, auto-heal actions, scan
  findings, cloud backup events, system notices) with category icons and
  relative timestamps. Refreshes every 30 s.

New backend endpoints:
- GET /api/dashboard/configuration - per-node feature status with locked/
  requiredTier markers so the frontend renders upgrade chips without extra
  calls. The endpoint sits after authGate and before the remote proxy so
  remote-node requests are transparently forwarded.
- GET /api/dashboard/recent-activity?limit=N - thin wrapper over
  DatabaseService.getNotificationHistory.
- GET /api/fleet/configuration - fleet-wide fan-out using the same
  Promise.allSettled dead-node-tolerant pattern as /fleet/overview.
  Exposed as the new "Status" tab on the Fleet page (after Snapshots).

Shared utilities:
- visibilityInterval and formatCount extracted to frontend/src/lib/utils.ts
  so the three polling hooks and two components share a single copy.

* docs(dashboard): fix stale alt text referencing removed historical charts

frontend/src/components/dashboard/useConfigurationStatus.ts

@@ -0,0 +1,84 @@
+import { useState, useEffect, useCallback, useRef } from 'react';
+import { useNodes } from '@/context/NodeContext';
+import { apiFetch } from '@/lib/api';
+import { visibilityInterval } from '@/lib/utils';
+
+export interface AgentStatus {
+  configured: boolean;
+  enabled: boolean;
+}
+
+export interface ConfigurationStatus {
+  tier: 'community' | 'paid';
+  variant: 'skipper' | 'admiral' | null;
+  notifications: {
+    agents: { discord: AgentStatus; slack: AgentStatus; webhook: AgentStatus };
+    alertRules: number;
+    routingRules: { count: number; enabledCount: number; locked: boolean; requiredTier: 'admiral' };
+  };
+  automation: {
+    autoHeal: { total: number; enabled: number };
+    autoUpdate: { enabled: number; total: number };
+    scheduledTasks: { total: number; enabled: number; locked: boolean; requiredTier: 'admiral' };
+    webhooks: { total: number; enabled: number; locked: boolean; requiredTier: 'skipper' };
+  };
+  security: {
+    mfaEnabled: boolean | null;
+    ssoEnabled: boolean;
+    ssoProvider: string | null;
+    scanPolicies: { total: number; enabled: number; locked: boolean; requiredTier: 'skipper' };
+  };
+  thresholds: {
+    cpuLimit: number;
+    ramLimit: number;
+    diskLimit: number;
+    dockerJanitorGb: number;
+    globalCrash: boolean;
+  };
+  backup: {
+    provider: 'disabled' | 'sencho' | 'custom';
+    autoUpload: boolean;
+    locked: boolean;
+    requiredTier: 'admiral';
+  };
+}
+
+export function useConfigurationStatus() {
+  const { activeNode } = useNodes();
+  const nodeId = activeNode?.id;
+  const nodeIdRef = useRef(nodeId);
+  useEffect(() => { nodeIdRef.current = nodeId; }, [nodeId]);
+
+  const [status, setStatus] = useState<ConfigurationStatus | null>(null);
+  const [loading, setLoading] = useState(true);
+
+  const fetchStatus = useCallback(async () => {
+    try {
+      const res = await apiFetch('/dashboard/configuration');
+      if (!res.ok) return;
+      const data = await res.json() as ConfigurationStatus;
+      setStatus(data);
+    } catch {
+      // Silent; stale data stays visible
+    } finally {
+      setLoading(false);
+    }
+  }, []);
+
+  useEffect(() => {
+    setStatus(null);
+    setLoading(true);
+    const currentNodeId = nodeId;
+    const guard = () => { if (nodeIdRef.current === currentNodeId) void fetchStatus(); };
+    guard();
+    return visibilityInterval(guard, 60_000);
+  }, [nodeId, fetchStatus]);
+
+  useEffect(() => {
+    const handler = () => void fetchStatus();
+    window.addEventListener('sencho:state-invalidate', handler);
+    return () => window.removeEventListener('sencho:state-invalidate', handler);
+  }, [fetchStatus]);
+
+  return { status, loading };
+}