feat(08-01): add diff viewer component and API client

- Add DiffResponse interface and getDiff method to configHistoryApi
- Create DiffViewer component with unified diff rendering
- Green highlighting for added lines, red for removed lines
- Blue styling for hunk headers, loading skeleton, error state

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/config/DiffViewer.tsx

@@ -0,0 +1,72 @@
+import { useQuery } from '@tanstack/react-query'
+import { X } from 'lucide-react'
+import { configHistoryApi } from '@/lib/api'
+
+interface DiffViewerProps {
+  tenantId: string
+  deviceId: string
+  snapshotId: string
+  onClose: () => void
+}
+
+function classifyLine(line: string): string {
+  if (line.startsWith('@@')) return 'bg-blue-900/20 text-blue-300'
+  if (line.startsWith('+++') || line.startsWith('---')) return 'text-text-muted'
+  if (line.startsWith('+')) return 'bg-green-900/30 text-green-300'
+  if (line.startsWith('-')) return 'bg-red-900/30 text-red-300'
+  return 'text-text-primary'
+}
+
+export function DiffViewer({ tenantId, deviceId, snapshotId, onClose }: DiffViewerProps) {
+  const { data: diff, isLoading, isError } = useQuery({
+    queryKey: ['config-diff', tenantId, deviceId, snapshotId],
+    queryFn: () => configHistoryApi.getDiff(tenantId, deviceId, snapshotId),
+  })
+
+  return (
+    <div className="rounded-lg border border-border bg-surface p-4">
+      {/* Header */}
+      <div className="flex items-center justify-between mb-3">
+        <div className="flex items-center gap-3">
+          <h3 className="text-sm font-medium text-muted-foreground">Config Diff</h3>
+          {diff && (
+            <span className="text-xs font-mono">
+              <span className="text-success">+{diff.lines_added}</span>
+              <span className="text-text-muted mx-0.5">/</span>
+              <span className="text-error">-{diff.lines_removed}</span>
+            </span>
+          )}
+        </div>
+        <button
+          onClick={onClose}
+          className="p-1 rounded hover:bg-elevated transition-colors text-text-muted hover:text-text-primary"
+        >
+          <X className="h-4 w-4" />
+        </button>
+      </div>
+
+      {/* Content */}
+      {isLoading ? (
+        <div className="space-y-2">
+          {Array.from({ length: 6 }).map((_, i) => (
+            <div key={i} className="h-4 bg-elevated rounded animate-pulse" style={{ width: `${60 + Math.random() * 40}%` }} />
+          ))}
+        </div>
+      ) : isError || !diff ? (
+        <div className="flex items-center justify-center py-6">
+          <span className="text-xs text-text-muted">No diff available.</span>
+        </div>
+      ) : (
+        <div className="overflow-auto max-h-[60vh] rounded border border-border bg-background">
+          <div className="font-mono text-xs whitespace-pre">
+            {diff.diff_text.split('\n').map((line, i) => (
+              <div key={i} className={`px-3 py-0.5 ${classifyLine(line)}`}>
+                {line || '\u00A0'}
+              </div>
+            ))}
+          </div>
+        </div>
+      )}
+    </div>
+  )
+}