Introduced reusable header control and improved MVVM architecture

[aurelm-w]

This PR refactors the Key Entries UI by extracting the toolbar responsibilities into a dedicated UI and VM, as requested in #11 .
This improves separation of concerns, reduces duplication and also makes the Key Entries view more maintainable and scalable.

• Encapsulates all toolbar-related UI inside KeyEntriesHeaderControl
• KeyEntriesHeaderViewModel acts as a proxy over KeyEntriesControlViewModel
• Commands and properties are forwarded through the header VM
• EditKeyStoreControlViewModel now propagates sticky header state across tabs and adds IsToolbarCollapsed tracking
• Header control scroll position updates VM state
• Added KeyEntryDeletionHandler to centralize deletion behavior
• Refactors dialog closed handling in UI layer
• Changed search action to MVVM command
• Reduced XAML complexity in KeyEntriesControl
• Improved refresh and loading state management

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 28 complexity · 0 duplication

Metric	Results
Complexity	28
Duplication	0

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