fix: show Bambu splash screen on Linux instead of black window

[BenJule]

Fixes #11532

On Linux the startup splash is currently a plain black window, because wxYield() is explicitly skipped on this platform right after the BBLSplashScreen is created. On wxGTK that call is what flushes the pending paint events — without it GTK never draws the splash before the long blocking startup work begins.

This change enables wxYield() on Linux too (it is harmless on the other platforms, which already ran it), raises the window and adds a Refresh()/Update() pair after the first SetText() so the status line actually becomes visible.

Before and after on Debian 13 / KDE Plasma:

I have been running this in my fork since May 2026 (Debian and Ubuntu builds, X11 and Wayland) with no regressions on Windows either.

[BenJule]

Additional verification after rebasing onto current master:

• Jenkins Linux PR build Feature Request: Purge to STL #37: SUCCESS
• Built commit: c3d8244db46a3cdf58c1f083f5bf104c7b583e05
• Debian Trixie build artifact was successfully generated and verified
• I downloaded and launched the exact Jenkins Feature Request: Purge to STL #37 artifact locally
• Manual startup test: PASS
• The Bambu splash screen is rendered immediately again instead of showing the black startup window

So the fix is now both compile-tested and manually verified with the CI-built artifact.

[tonghao-bbl]

@lanewei120 LGTM