feat: complete Phase 5 - integration & main entry point

Add main application entry point and integration layer:

src/index.ts (415 lines):
- Main entry point that orchestrates entire application lifecycle
- Initialize data directory and load configuration
- Parse CLI arguments for connection modes and config overrides
- Initialize all managers (Config, Context, Connection, Backend)
- Initialize all services (Polling, Camera, RTSP, Spoolman, Monitoring)
- Connect to printers based on CLI mode (last-used, all-saved, explicit, no-printers)
- Start WebUI server and verify successful startup
- Setup event forwarding from polling coordinator to WebUI
- Start polling for all connected printers
- Initialize camera proxies for each context
- Handle graceful shutdown on SIGINT/SIGTERM

src/utils/HeadlessArguments.ts (220 lines):
- CLI argument parser for standalone server configuration
- Support 4 connection modes:
  * --last-used: Connect to last used printer
  * --all-saved-printers: Connect to all saved printers
  * --printers="IP:TYPE:CODE,...": Connect to specific printers
  * --no-printers: Start WebUI without printer connections
- Support config overrides:
  * --webui-port=PORT: Override WebUI port
  * --webui-password=PASSWORD: Override WebUI password
- Argument validation with detailed error messages

src/utils/setup.ts (75 lines):
- Data directory initialization and validation
- Ensure data directory exists and is writable
- Support DATA_DIR environment variable for custom location
- Default location: ./data

tsconfig.json:
- Add src/index.ts to include array for compilation

Build output:
- dist/index.js: Main entry point (14.9KB compiled)
- dist/utils/HeadlessArguments.js: CLI parser (5.2KB)
- dist/utils/setup.js: Data directory setup (3.7KB)

Type check: 0 errors
Build: Success
Phase 5 Integration & Testing: Complete

Author: claude