Add alphabetical sorting to TreeView items

[Copilot]

This PR implements alphabetical sorting for all TreeView items across the three-level hierarchy in the Azure Key Vault Explorer.

Changes Made

📁 New SortService Utility

Created a new SortService class that provides sorting methods for all TreeView collection types:

• Subscriptions: Sorted alphabetically with "Quick Access" always at the top
• Resource Groups: Sorted alphabetically by display name
• Key Vaults: Sorted alphabetically by vault name

🔍 Enhanced Filtering

Updated FilterService to maintain alphabetical order in search results, ensuring filtered items are always displayed in sorted order.

🌳 TreeView Integration

Applied sorting at all key data loading points:

• Initial subscription loading in GetAvailableKeyVaults()
• Resource group expansion in KvSubscriptionModel_PropertyChanged()
• Key vault expansion in KvResourceGroupNode_PropertyChanged()
• Quick Access item management (PinVaultToQuickAccess())

Technical Details

The implementation ensures:

• ✅ Performance: Sorting only occurs when collections are modified
• ✅ Null Safety: Graceful handling of null/empty display names
• ✅ Special Cases: "Quick Access" subscription always appears first
• ✅ Search Compatibility: Filtered results maintain alphabetical order
• ✅ Minimal Impact: No breaking changes to existing functionality

Before/After

Before: TreeView items appeared in the order they were loaded from Azure
After: All items are sorted alphabetically at every level, improving navigation and user experience

The sorting is applied consistently across:

1. Level 1: Subscriptions (by SubscriptionDisplayName)
2. Level 2: Resource Groups (by ResourceGroupDisplayName)
3. Level 3: Key Vaults (by Data.Name)

Fixes #128.

Warning

Firewall rules blocked me from connecting to one or more addresses

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• av-build-tel-api-v1.avaloniaui.net
  • Triggering command: dotnet exec --runtimeconfig /home/REDACTED/.nuget/packages/avalonia.buildservices/0.0.31/tools/netstandard2.0/runtimeconfig.json /home/REDACTED/.nuget/packages/avalonia.buildservices/0.0.31/tools/netstandard2.0/Avalonia.BuildServices.Collector.dll (dns block)
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If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

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