Tray icon missing when the manager service auto-launches the UI at boot/logon (manual launch works) - v0.10.3

[radimrucil-elexim]

Describe the bug

After Windows starts, the Pangolin UI process is launched automatically (it is
visible in Task Manager as Pangolin.exe), but no system tray icon appears
and there is no window. The process just sits there "invisible". If I then start
Pangolin manually (Start menu / exe), the tray icon appears correctly and
everything works.

This looks distinct from #13 (which was standard-user accounts being
excluded by the isAdmin/isOperator check and was fixed in 0.6.0). In my case
the UI process is spawned and a manual launch does work, so authorization is
fine — this is a timing/race problem with the auto-launched instance only.

Environment

• Pangolin Windows client 0.10.3 (MSI: pangolin-amd64-0.10.3.msi)
• Windows 11 Pro 64-bit (build 10.0.26200)
• Per-machine install to C:\Program Files\Pangolin\

Steps to reproduce

1. Install Pangolin and let it run (manager service installed, UI enabled).
2. Reboot / log in.
3. Observe: Pangolin.exe is running (Task Manager) but there is no tray icon
   and no window.
4. Manually launch Pangolin -> tray icon appears and works normally.

Likely root cause

Inspecting Pangolin.exe shows the manager service launches the UI via
managers.launchUIProcess using WTSGetActiveConsoleSessionId +
WTSQueryUserToken + CreateProcessAsUser. At boot/logon this runs before the
shell's notification area is ready, so the initial Shell_NotifyIcon(NIM_ADD)
fails silently while the process keeps running. A later manual launch (shell
already up) succeeds. There is a UI launch: retry attempt path, but it is not
sufficient in this scenario.

Suggested fix

Make the tray UI resilient to being started before the shell:

1. Handle the TaskbarCreated registered window message and (re-)add the
   tray icon when it arrives. This is the canonical Win32 pattern and also fixes
   the icon vanishing after an Explorer restart.
2. Optionally, retry Shell_NotifyIcon(NIM_ADD) with backoff for longer / until
   it succeeds, or wait for explorer.exe / the shell window to be present
   before the first add.