[client] UI refactor (#6069)

Refactor UI

---------

Co-authored-by: Eduard Gert <kontakt@eduardgert.de>
Co-authored-by: braginini <bangvalo@gmail.com>
Co-authored-by: Pascal Fischer <32096965+pascal-fischer@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: riccardom <riccardomanfrin@gmail.com>

client/ui/recenter_linux.go

@@ -0,0 +1,13 @@
+//go:build linux && !(linux && 386)
+
+package main
+
+// recenterOnShowPredicate returns a per-show predicate; re-centering is only
+// needed under the minimal-WM / in-process-XEmbed-tray environment, which neither
+// centers small windows nor restores position across hide -> show. Evaluated per
+// show, not at startup, because the XEmbed tray can appear after the UI starts
+// (panel and autostarted app race at login); xembedTrayAvailable is a cheap,
+// side-effect-free probe safe to call repeatedly.
+func recenterOnShowPredicate() func() bool {
+	return xembedTrayAvailable
+}