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>

CHANGELOG.md

@@ -7,6 +7,49 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [2.34.0] - 2026-02-07
+
+### Added
+
+- **MCP Apps**: Rich HTML UIs rendered by MCP hosts alongside tool results via `_meta.ui` and the MCP resources protocol
+  - Server-side UI module (`src/mcp/ui/`) with tool-to-UI mapping and `_meta.ui` injection
+  - `UIAppRegistry` static class for loading and serving self-contained HTML apps
+  - `UI_APP_CONFIGS` mapping tools to their corresponding UI apps
+
+- **Operation Result UI**: Visual summary for workflow operation tools
+  - Status badge, operation type, workflow details card
+  - Expandable sections for nodes added, modified, and removed
+  - Mapped to: `n8n_create_workflow`, `n8n_update_full_workflow`, `n8n_update_partial_workflow`, `n8n_delete_workflow`, `n8n_test_workflow`, `n8n_autofix_workflow`, `n8n_deploy_template`
+
+- **Validation Summary UI**: Visual summary for validation tools
+  - Valid/invalid badge with error and warning counts
+  - Expandable error list with type, property, message, and fix
+  - Expandable warning list and suggestions
+  - Mapped to: `validate_node`, `validate_workflow`, `n8n_validate_workflow`
+
+- **React + Vite Build Pipeline** (`ui-apps/`):
+  - React 19, Vite 6, vite-plugin-singlefile for self-contained HTML output
+  - Shared component library: Card, Badge, Expandable
+  - `useToolData` hook for reading data from `window.__MCP_DATA__` or embedded JSON
+  - n8n-branded dark theme with CSS custom properties
+  - Per-app builds via `APP_NAME` environment variable
+
+- **MCP Resources Protocol**: Server now exposes `resources` capability
+  - `ListResources` handler returns available UI apps
+  - `ReadResource` handler serves self-contained HTML via `n8n-mcp://ui/{id}` URIs
+
+- **New Scripts**:
+  - `build:ui`: Build UI apps (`cd ui-apps && npm install && npm run build`)
+  - `build:all`: Build UI apps then server (`npm run build:ui && npm run build`)
+
+### Changed
+
+- **MCP Server**: Added `resources: {}` to server capabilities alongside existing `tools: {}`
+- **Tool Responses**: Tools with matching UI apps now include `_meta.ui.app` URI pointing to their visual representation
+- **Graceful Degradation**: Server starts and operates normally without `ui-apps/dist/`; UI metadata is only injected when HTML is available
+
+Conceived by Romuald Czlonkowski - https://www.aiadvisors.pl/en
+
 ## [2.33.6] - 2026-02-06
 
 ### Changed