The XEmbed tray (panel) can come up after the autostarted UI on minimal
WMs, so the single startup probe added in #6320 could miss a tray that
appears a second or two later, leaving the icon silently absent. Re-probe
for a ~10s grace period in a goroutine, claiming the watcher as soon as a
tray shows up; back off cleanly if none ever appears (headless/Wayland).
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