fix(gui): reopen Connect to Radio dialog reliably under xcb (#4725)

[g4ivv]

Summary

Investigating #4725's native-Wayland panadapter/waterfall throttling led to
QT_QPA_PLATFORM=xcb as the practical workaround — native Wayland's
render-pipeline stalls on this platform have no clean in-app fix (see the
README note added here; a lot was tried: native-window isolation, a
GPU-presentation watchdog, periodic surface recreation — all either
ineffective or trading the freeze for a recurring visible flash. This looks
like a genuine Qt wayland-platform-plugin defect, not something AetherSDR's
own code controls).

But xcb had its own defect: once connected, the "Connect to Radio" dialog
auto-hides, and reopening it afterward (station-label double-click or the
menu item) called show() successfully — isVisible() reported true —
yet nothing appeared, not even in the window switcher, making it impossible
to disconnect. That made the workaround worse than the problem it solves.

Root cause

Confirmed with xprop/xwininfo: the second show(), following a real
hide(), left the window wedged in the ICCCM Withdrawn WM_STATE under
Mutter/XWayland — genuinely unmapped, absent from _NET_CLIENT_LIST_STACKING,
not just unfocused or stacked behind the main window. The very first show
(before ever being hidden) always worked, which is why this only surfaced
after connecting. Not reproduced under native Wayland, which has no ICCCM
Withdrawn/Normal state machine to get stuck in.

Fix

showConnectionDialog() now destroys and recreates the native window before
every re-show rather than re-mapping the stale withdrawn one.
raise()/activateWindow() also moved to a deferred event-loop turn, since
both call sites reach this function from inside a live X11 input/grab context
(a menu popup's own grab teardown, or the station label's double-click still
being dispatched).

Test plan

• Confirmed fixed against real FlexRadio hardware (not just the demo
  simulator): both the menu item and the station-label double-click
  reliably reopen the dialog after connecting, repeatedly.
• xprop/xwininfo confirm WM_STATE: Normal and correct
  _NET_CLIENT_LIST_STACKING membership across repeated show/hide cycles.
• Existing build is unaffected on other platforms — the fix only touches
  the xcb-observed re-show path and is a no-op on the very first show.

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