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>

ui-apps/src/shared/components/Expandable.tsx

@@ -0,0 +1,36 @@
+import React from 'react';
+
+interface ExpandableProps {
+  title: string;
+  count?: number;
+  defaultOpen?: boolean;
+  children: React.ReactNode;
+}
+
+export function Expandable({ title, count, defaultOpen = false, children }: ExpandableProps) {
+  return (
+    <details open={defaultOpen} style={{
+      marginBottom: '8px',
+      border: '1px solid var(--n8n-border)',
+      borderRadius: 'var(--n8n-radius)',
+      overflow: 'hidden',
+    }}>
+      <summary style={{
+        padding: '10px 14px',
+        cursor: 'pointer',
+        fontSize: '13px',
+        fontWeight: 500,
+        background: 'var(--n8n-bg-card)',
+        userSelect: 'none',
+      }}>
+        {title}
+        {count !== undefined && (
+          <span style={{ marginLeft: '8px', color: 'var(--n8n-text-muted)' }}>({count})</span>
+        )}
+      </summary>
+      <div style={{ padding: '12px 14px' }}>
+        {children}
+      </div>
+    </details>
+  );
+}