fix(macos): use CGRequestScreenCaptureAccess instead of grabWindow for permission request

[Mitnitsky]

Problem

On modern macOS (especially 15+), Flameshot re-triggers the Screen Recording permission dialog on every launch. The user grants the permission, but the next time the app starts it prompts again.

Root cause

The current code uses QScreen::grabWindow() as a hack to trigger the TCC permission prompt. This performs an actual screen capture on every launch, which modern macOS interprets as a new capture attempt rather than a no-op — re-triggering the permission dialog even when already granted.

Fix

Replace the grabWindow() hack with the proper CoreGraphics APIs:

• CGPreflightScreenCaptureAccess() — checks if permission is already granted (no prompt)
• CGRequestScreenCaptureAccess() — shows the system dialog only if not yet granted

These APIs have been available since macOS 10.15 (Catalina) and are the documented way to handle Screen Recording permissions.

[Mitnitsky]

#3463 probably related

[borgmanJeremy]

This looks good to me. I want to do some testing here but I plan to merge for the actual v14 release.

[borgmanJeremy]

I do not get the screen permission prompt every launch, but I almost always test a from source build. Does the issue only happen when using a .dmg built in CI?