feat(client): system tray on Windows, and a clearer tray icon

The tray's Start/Stop already drove nodeService.status/stop/restart, which
had full win32 branches for both startup modes from the Node page work --
so enabling it on Windows is widening two platform gates (the `tray`
capability in preload.js, the window:minimize-to-tray handler in main.js),
not new logic.

Replaced the tray icon: the previous white chevron-in-a-box read as an
envelope at tray size. New icon is a small "M" drawn as mesh nodes and
edges, echoing the app icon's own motif, in the brand blue instead of
plain white so it stays legible on both light and dark taskbars/panels.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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