feat(nodes): add capability-based node compatibility negotiation (#350)

* feat(nodes): add capability-based node compatibility negotiation

Each Sencho instance now exposes /api/meta with its version and supported
capabilities. When the user switches nodes, the frontend fetches this
metadata and disables features the remote node doesn't support via a
CapabilityGate overlay. Version is shown in the node switcher dropdown
and connection test results.

- Backend: CapabilityRegistry with static capability list and fetchRemoteMeta helper
- Backend: /api/meta (public) and /api/nodes/:id/meta (auth) endpoints
- Frontend: NodeContext enhanced with per-node meta caching (5min TTL)
- Frontend: CapabilityGate component with typed Capability union
- Frontend: 13 features wrapped with capability gates
- Docs: node-compatibility.mdx + OpenAPI spec updates

* fix(nodes): revert to require() for package.json version reading

The static import fails in the Docker multi-stage build because the
root package.json is not copied into the backend-builder stage. The
require() call resolves at runtime when the file is available.

frontend/src/components/CapabilityGate.tsx

@@ -0,0 +1,35 @@
+import { type ReactNode } from 'react';
+import { Unplug } from 'lucide-react';
+import { useNodes } from '@/context/NodeContext';
+import type { Capability } from '@/lib/capabilities';
+
+interface CapabilityGateProps {
+  capability: Capability;
+  featureName?: string;
+  children: ReactNode;
+}
+
+export function CapabilityGate({ capability, featureName = 'This feature', children }: CapabilityGateProps) {
+  const { hasCapability, activeNode, activeNodeMeta } = useNodes();
+
+  if (hasCapability(capability)) return <>{children}</>;
+
+  const nodeName = activeNode?.name ?? 'this node';
+  const versionHint = activeNodeMeta?.version
+    ? `${nodeName} is running v${activeNodeMeta.version}`
+    : `${nodeName} does not support this capability`;
+
+  return (
+    <div className="relative">
+      <div className="opacity-40 pointer-events-none select-none blur-[2px]">
+        {children}
+      </div>
+      <div className="absolute inset-0 flex items-start justify-center pt-8">
+        <div className="flex items-center gap-2 px-3 py-1.5 rounded-full bg-muted/80 border border-border text-muted-foreground text-xs">
+          <Unplug className="w-3 h-3" strokeWidth={1.5} />
+          {featureName} is not available — {versionHint}
+        </div>
+      </div>
+    </div>
+  );
+}