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.

docs/openapi.yaml

@@ -543,6 +543,33 @@ paths:
                     type: number
                     description: Process uptime in seconds.
 
+  /api/meta:
+    get:
+      operationId: getMeta
+      tags: [Health]
+      summary: Instance metadata
+      description: Returns this Sencho instance's version and list of supported capabilities. No authentication required. Used by nodes to negotiate feature compatibility.
+      security: []
+      responses:
+        "200":
+          description: Instance metadata.
+          content:
+            application/json:
+              schema:
+                type: object
+                required: [version, capabilities]
+                properties:
+                  version:
+                    type: string
+                    description: Sencho version (semver).
+                    example: "0.32.0"
+                  capabilities:
+                    type: array
+                    description: List of feature capabilities this instance supports.
+                    items:
+                      type: string
+                    example: ["stacks", "containers", "fleet", "auto-updates", "host-console"]
+
   # ── Stacks ──────────────────────────────────────────────
   /api/stacks:
     get:
@@ -1767,6 +1794,39 @@ paths:
         "500":
           $ref: "#/components/responses/InternalError"
 
+  /api/nodes/{id}/meta:
+    get:
+      operationId: getNodeMeta
+      tags: [Nodes]
+      summary: Get node capabilities
+      description: Returns the version and supported capabilities for a specific node. For local nodes, returns this instance's own metadata. For remote nodes, relays the response from the remote instance's `/api/meta` endpoint. Returns `{ version: null, capabilities: [] }` if the remote node is offline or doesn't support the meta endpoint.
+      parameters:
+        - $ref: "#/components/parameters/idPath"
+      responses:
+        "200":
+          description: Node capability metadata.
+          content:
+            application/json:
+              schema:
+                type: object
+                required: [version, capabilities]
+                properties:
+                  version:
+                    type: string
+                    nullable: true
+                    description: Sencho version of the node, or null if unavailable.
+                    example: "0.31.0"
+                  capabilities:
+                    type: array
+                    description: List of feature capabilities the node supports. Empty array if unavailable.
+                    items:
+                      type: string
+                    example: ["stacks", "containers", "fleet"]
+        "401":
+          $ref: "#/components/responses/Unauthorized"
+        "404":
+          $ref: "#/components/responses/NotFound"
+
   # ── Fleet ───────────────────────────────────────────────
   /api/fleet/overview:
     get: