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/styles/theme.css

@@ -0,0 +1,32 @@
+:root {
+  --n8n-primary: #ff6d5a;
+  --n8n-primary-light: #ff8a7a;
+  --n8n-success: #17bf79;
+  --n8n-success-light: #e8f9f0;
+  --n8n-warning: #f59e0b;
+  --n8n-warning-light: #fef3cd;
+  --n8n-error: #ef4444;
+  --n8n-error-light: #fee2e2;
+  --n8n-info: #3b82f6;
+  --n8n-info-light: #dbeafe;
+  --n8n-bg: #1a1a2e;
+  --n8n-bg-card: #252540;
+  --n8n-text: #e0e0e0;
+  --n8n-text-muted: #9ca3af;
+  --n8n-border: #374151;
+  --n8n-radius: 8px;
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
+}
+
+* {
+  margin: 0;
+  padding: 0;
+  box-sizing: border-box;
+}
+
+body {
+  background: var(--n8n-bg);
+  color: var(--n8n-text);
+  line-height: 1.5;
+  padding: 16px;
+}