Refactor MiscTab to MVVM and reorganize ViewModels

[ebuzerdrmz44]

Description

This PR addresses the ViewModel extraction for the MiscTab and introduces structural organization for future ViewModels.

Related Issue

#2361 Phase 5 Partially

Motivation and Context

This is part of the ongoing architectural effort to decouple UI from data access logic. Moving the filtering and database logic into ViewModels makes the tabs easier to test without a full PyQt GUI context and reduces code duplication. Creating the viewmodels directory ensures the codebase remains organized as the remaining tabs are refactored during GSoC.

How Has This Been Tested?

Added complete unit tests for MiscTabViewModel in tests/unit/test_misc_tab_viewmodel.py to ensure it successfully retrieves groups, saves settings, and filters legacy keys correctly.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read the CONTRIBUTING guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the Developer Certificate of Origin.