Add menu bar and system tray mode

[rafiki270]

Summary

• Add a saved menu bar-only option on macOS and system tray-only option on Windows through the native application menu.
• Hide the window and Dock/taskbar entry while keeping the local board service running. Provide Show and Quit commands and restore normal window mode when disabled.
• Keep the preference in a separate, atomically written desktop.json file and restore it on launch; damaged preferences open normally.
• Add lifecycle coverage and document setup, behavior, and native verification.

Testing

• Mac ARM64: lint, 16 backend/MCP tests, five source UI flows, ZIP build, packaged MCP, and five packaged UI flows passed.
• Windows x64: lint, 16 backend/MCP tests, five source UI flows, NSIS build, packaged MCP, and five packaged UI flows passed on the native host.
• Three-platform CI passed for implementation commit 64a1e97, including sandboxed Linux source/packaged flows. The tray-specific test applies to macOS/Windows only.
• Rebuilt Mac application launched outside the harness; native menu toggling, saved preference, Quit, and relaunch verified.
• Signing and installer installation/upgrade were not tested. Linux keeps normal window behavior.