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/lib/capabilities.ts

@@ -0,0 +1,25 @@
+/** Must stay in sync with backend/src/services/CapabilityRegistry.ts */
+export const CAPABILITIES = [
+  'stacks',
+  'containers',
+  'resources',
+  'templates',
+  'global-logs',
+  'system-stats',
+  'fleet',
+  'auto-updates',
+  'labels',
+  'webhooks',
+  'network-topology',
+  'notifications',
+  'notification-routing',
+  'host-console',
+  'audit-log',
+  'scheduled-ops',
+  'sso',
+  'api-tokens',
+  'users',
+  'registries',
+] as const;
+
+export type Capability = (typeof CAPABILITIES)[number];