Add real background/tray mode (don't kill agent sessions on window close)

**Problem**

Closing the Buzz Desktop main window (the X button) terminates `buzz-desktop.exe`
and all of its child `buzz-acp.exe` agent-session processes immediately.
Minimizing works fine and keeps everything alive, but there's no tray icon /
"minimize to tray" registration, so closing the window is the only way to
"get it out of the way" -- and it kills every running agent session.

**Steps to reproduce**

1. Have one or more agent sessions active in Buzz Desktop.
2. Close the main window with the X button (not minimize).
3. Observe: `buzz-desktop.exe` and all `buzz-acp.exe` child processes exit;
agent sessions stop.

**Expected**

A background/tray mode (similar to Tailscale, OneDrive, etc.) so closing the
window doesn't kill in-progress agent work -- e.g. a system tray icon with the
window continuing to run minimized/hidden until the user explicitly quits.

**Environment**

- Windows 11
- Confirmed via Task Manager: no tray icon registered for buzz-desktop;
buzz-acp.exe processes are direct children of buzz-desktop.exe and exit
when the parent window is closed.

Start by tracing Buzz Desktop's Windows close handling and the lifetime relationship between buzz-desktop.exe and its buzz-acp.exe child processes. Reproduce the behavior with an active agent session, then verify that closing the window leaves a tray or hidden background process running while an explicit quit ends the desktop and agent processes.

Written by the indexing model from the issue text.