feat(M3): Host Management, Groups, Users, CIDR Discovery

- pm-core::models: Host, HostSummary, Group, User, DiscoveryResult
  types + request payloads for all CRUD operations
- pm-core::audit: Tamper-evident hash-chained audit log writer
  (SHA-256 chain, non-fatal, covers all M3 events)
- pm-web/routes/hosts: Full host CRUD with RBAC scoping;
  FQDN DNS resolution on registration; host↔group membership;
  operator group-scoped access enforcement; audit on register/remove
- pm-web/routes/groups: Full group CRUD; host↔group and user↔group
  membership management; admin-only create/delete/update
- pm-web/routes/users: Full user CRUD (admin); current user profile;
  password hashing (Argon2id); role management; session revocation
- pm-web/routes/discovery: CIDR scan with bounded concurrency
  (128 workers), TCP probe with 2s timeout, reverse DNS lookup,
  scan results table, register-from-discovery flow with audit log
- Frontend: HostsPage (filterable table with health chips),
  HostDetailPage, GroupsPage (create/delete dialog),
  UsersPage (create/revoke sessions)
- App.tsx updated with all M3 routes wired to real pages
- cargo check --workspace: zero errors

Closes M3.

frontend/src/pages/HostDetailPage.tsx

@@ -0,0 +1,41 @@
+import { useEffect, useState } from 'react'
+import { useParams, useNavigate } from 'react-router-dom'
+import { Alert, Box, Button, Chip, CircularProgress, Container, Divider, Grid, Paper, Typography } from '@mui/material'
+import { ArrowBack } from '@mui/icons-material'
+import { apiClient } from '../api/client'
+
+export default function HostDetailPage() {
+  const { id } = useParams<{ id: string }>()
+  const navigate = useNavigate()
+  const [host, setHost] = useState<Record<string, unknown> | null>(null)
+  const [loading, setLoading] = useState(true)
+  const [error, setError] = useState<string | null>(null)
+
+  useEffect(() => {
+    apiClient.get(`/hosts/${id}`)
+      .then(r => setHost(r.data))
+      .catch(() => setError('Host not found or access denied.'))
+      .finally(() => setLoading(false))
+  }, [id])
+
+  if (loading) return <Box display="flex" justifyContent="center" mt={8}><CircularProgress /></Box>
+  if (error) return <Container sx={{ mt: 4 }}><Alert severity="error">{error}</Alert></Container>
+
+  return (
+    <Container maxWidth="lg" sx={{ mt: 3 }}>
+      <Button startIcon={<ArrowBack />} onClick={() => navigate('/hosts')} sx={{ mb: 2 }}>Back to Hosts</Button>
+      <Paper sx={{ p: 3 }}>
+        <Typography variant="h5" fontWeight={700} mb={2}>{String(host?.fqdn ?? '')}</Typography>
+        <Divider sx={{ mb: 2 }} />
+        <Grid container spacing={2}>
+          {host && Object.entries(host).map(([k, v]) => v !== null && v !== '' ? (
+            <Grid item xs={12} sm={6} md={4} key={k}>
+              <Typography variant="caption" color="text.secondary" display="block">{k.replace(/_/g, ' ').toUpperCase()}</Typography>
+              <Typography variant="body2">{String(v)}</Typography>
+            </Grid>
+          ) : null)}
+        </Grid>
+      </Paper>
+    </Container>
+  )
+}