fix: align MCP Apps with official ext-apps spec (#574)

* 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>

* fix: align MCP Apps with official ext-apps spec

Update URI scheme from n8n-mcp://ui/ to ui://n8n-mcp/ per MCP spec.
Move _meta.ui.resourceUri to tool definitions (tools/list) instead of
tool call responses. Rewrite UI apps hook to use @modelcontextprotocol/ext-apps
App class instead of window.__MCP_DATA__.

Conceived by Romuald Czlonkowski - 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/registry.ts

@@ -60,6 +60,21 @@ export class UIAppRegistry {
     return Array.from(this.entries.values());
   }
 
+  /**
+   * Enrich tool definitions with _meta.ui.resourceUri for tools that have
+   * a matching UI app. Per MCP ext-apps spec, this goes on the tool
+   * definition (tools/list), not the tool call response.
+   */
+  static injectToolMeta(tools: Array<{ name: string; [key: string]: any }>): void {
+    if (!this.loaded) return;
+    for (const tool of tools) {
+      const entry = this.toolIndex.get(tool.name);
+      if (entry && entry.html) {
+        tool._meta = { ui: { resourceUri: entry.config.uri } };
+      }
+    }
+  }
+
   /** Reset registry state. Intended for testing only. */
   static reset(): void {
     this.entries.clear();