Fix #2445: Add missing WindowMaximizeButtonHint to main window flags

[Denay2468]

What this PR does

The maximize button in Vorta's main window title bar was not working (especially on Linux).
setWindowFlags only included WindowCloseButtonHint and WindowMinimizeButtonHint, missing WindowMaximizeButtonHint.

This PR adds the missing flag so the maximize button becomes functional again.

Related Issue

• Fixes Cannot maximize the main window #2445

Testing

• Manually tested on Linux (GNOME)
• Maximize / minimize / close all work normally
• No changes to Windows or macOS behavior

Checklist

• I have read the CONTRIBUTING guide
• My code follows the code style of this project
• All new and existing tests passed

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This is my third contribution in the last few days:

• fix: include ExclusionModel in profile export/import #2471 (merged March 24) — Fixed excludes list being dropped on profile export
• fix: guard darwin network test import behind platform check #2470 — Fixed Windows test collection crash

Applying for Google Summer of Code 2026 with Borg Collective (Vorta). Happy to take on bigger tasks after this small visible fix!

Ready for review! 🚀