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/Badge.tsx

@@ -0,0 +1,32 @@
+import React from 'react';
+
+type BadgeVariant = 'success' | 'warning' | 'error' | 'info';
+
+interface BadgeProps {
+  variant: BadgeVariant;
+  children: React.ReactNode;
+}
+
+const variantStyles: Record<BadgeVariant, { bg: string; color: string }> = {
+  success: { bg: 'var(--n8n-success-light)', color: 'var(--n8n-success)' },
+  warning: { bg: 'var(--n8n-warning-light)', color: 'var(--n8n-warning)' },
+  error: { bg: 'var(--n8n-error-light)', color: 'var(--n8n-error)' },
+  info: { bg: 'var(--n8n-info-light)', color: 'var(--n8n-info)' },
+};
+
+export function Badge({ variant, children }: BadgeProps) {
+  const style = variantStyles[variant];
+  return (
+    <span style={{
+      display: 'inline-block',
+      padding: '2px 10px',
+      borderRadius: '12px',
+      fontSize: '12px',
+      fontWeight: 600,
+      background: style.bg,
+      color: style.color,
+    }}>
+      {children}
+    </span>
+  );
+}