feat: MCP Apps - rich HTML UIs for tool results (#573)

* feat: add MCP Apps with rich HTML UIs for tool results

Add MCP Apps infrastructure that allows MCP hosts like Claude Desktop
to render rich HTML UIs alongside tool results via `_meta.ui` and the
MCP resources protocol.

- Server-side UI module (src/mcp/ui/) with UIAppRegistry, tool-to-UI
  mapping, and _meta.ui injection into tool responses
- React + Vite build pipeline (ui-apps/) producing self-contained HTML
  per app using vite-plugin-singlefile
- Operation Result UI for workflow CRUD tools (create, update, delete,
  test, autofix, deploy)
- Validation Summary UI for validation tools (validate_node,
  validate_workflow, n8n_validate_workflow)
- Shared component library (Card, Badge, Expandable) with n8n dark theme
- MCP resources protocol support (ListResources, ReadResource handlers)
- Graceful degradation when ui-apps/dist/ is not built
- 22 unit tests across 3 test files

Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en

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

* test: improve MCP Apps test coverage and add security hardening

- Expand test suite from 22 to 57 tests across 3 test files
- Add UIAppRegistry.reset() for proper test isolation between tests
- Replace some fs mocks with real temp directory tests in registry
- Add edge case coverage: empty strings, pre-load state, double load,
  malformed URIs, duplicate tool patterns, empty HTML files
- Add regression tests for specific tool-to-UI mappings
- Add URI format consistency validation across all configs
- Improve _meta.ui injection tests with structuredContent coexistence
- Coverage: statements 79.4% -> 80%, lines 79.4% -> 80%

Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/mcp/ui/app-configs.ts

@@ -0,0 +1,32 @@
+import type { UIAppConfig } from './types';
+
+export const UI_APP_CONFIGS: UIAppConfig[] = [
+  {
+    id: 'operation-result',
+    displayName: 'Operation Result',
+    description: 'Visual summary of workflow operations (create, update, delete, test)',
+    uri: 'n8n-mcp://ui/operation-result',
+    mimeType: 'text/html',
+    toolPatterns: [
+      'n8n_create_workflow',
+      'n8n_update_full_workflow',
+      'n8n_update_partial_workflow',
+      'n8n_delete_workflow',
+      'n8n_test_workflow',
+      'n8n_autofix_workflow',
+      'n8n_deploy_template',
+    ],
+  },
+  {
+    id: 'validation-summary',
+    displayName: 'Validation Summary',
+    description: 'Visual summary of node and workflow validation results',
+    uri: 'n8n-mcp://ui/validation-summary',
+    mimeType: 'text/html',
+    toolPatterns: [
+      'validate_node',
+      'validate_workflow',
+      'n8n_validate_workflow',
+    ],
+  },
+];