🐛 auto-show main window on first launch for all desktop platforms

[guiyanakuang]

Closes #4696

Summary

On Windows and Linux the first launch showed no window at all: the main window starts hidden and the only first-launch auto-show call lived in the macOS tray view (MacTrayView.Tray()). New users saw nothing but a (possibly overflow-collapsed) tray icon and perceived the app as "not opening".

This PR lifts the first-launch logic into the platform-agnostic CrossPasteWindows composable:

• firstLaunch && !firstLaunchCompleted → showMainWindow(WindowTrigger.SYSTEM) + setFirstLaunchCompleted(true) now runs on macOS, Windows, and Linux
• The duplicate block and now-unused injections are removed from MacTrayView

Side effect fixed

setFirstLaunchCompleted(true) was previously macOS-only, so firstLaunchCompleted stayed false forever on Windows/Linux and the blinking TutorialButton (gated on firstLaunchCompleted && config.showTutorial) never appeared there. It now works on all platforms.

Behavior notes

• The effect is placed after the tray section in CrossPasteWindows, preserving the previous macOS ordering (tray init before window show)
• On macOS the window now shows even if native tray init throws, which previously would have skipped it — a strict improvement
• Non-first launches are unchanged: the app still starts hidden in the tray

Testing

• ./gradlew ktlintFormat clean
• ./gradlew app:compileKotlinDesktop passes
• ./gradlew app:desktopTest passes