feat: introduce MCP server, cross-platform runtime, and Showcase UI reorganization

- Showcase UI Reorganization:
  * Move web UI dashboard from root directory into apps/showcase_ui/ for cleaner project structure
  * Add 'artemis ui' CLI command (artemis/interfaces/cli/commands/ui.py) and launcher scripts for the interactive showcase console
- Standalone MCP Server & Notifiers (mcp_server/):
  * Implement dedicated MCP server package with mobile automation tools (device_state, inspect_trace, task_manager, task_runner)
  * Add multi-channel notifiers (AgentAPI, Desktop notifications, Webhooks, File output, and Composite routing)
  * Introduce background task runner and trace store for autonomous mobile execution monitoring
- Cross-Platform Runtime & Toolchain Support:
  * Add artemis/platform/ with OS-specific implementations for Linux, macOS (Darwin), and Windows
  * Add artemis/runtime/supervisor.py for resilient process and agent runtime supervision
  * Add artemis/toolchain/ with dependency resolution and SDK descriptors
  * Add cross-platform launcher scripts (start.sh, start.bat) and installer scripts (scripts/install_deps.*)
- Core Enhancements & Diagnostics:
  * Enhance coordinate normalization, OCR API fallback, and perception pipeline
  * Add credential validation (credentials_validator.py) and system diagnostics
  * Update bilingual documentation (README.md, README_CN.md) and architecture diagrams
- Test Suite Expansion:
  * Add integration tests for cloud gateway, remote ADB, and Artemis login
  * Add unit tests covering core diagnostics, platform detection, toolchain, credentials validator, optional OCR, and MCP tools

artemis/engine/reactive_runner.py

@@ -23,6 +23,7 @@ from artemis.core.state import ExecutionContextState, ExecutionStatus, StepRecor
 from artemis.drivers.mock.mock_driver import MockDeviceDriver
 from artemis.engine.base_runner import BaseRunner
 from artemis.graph.state import State
+from artemis.platform import platform
 from artemis.utils.logger import get_logger
 
 logger = get_logger(__name__)
@@ -57,7 +58,7 @@ class ReactiveRunner(BaseRunner):
                 if artemis_ctx is None:
                     artemis_ctx = ArtemisContext(
                         device=DeviceContext(
-                            host_platform="LINUX",
+                            host_platform=platform.os_type.name,
                             device_serial=self.driver.device_id,
                         )
                     )