refactor: use collection expressions and is-null pattern in remaining sites

[laurentiu021]

Summary

Byte-identical .NET 10 / C# idiom cleanups — the remaining mechanical modernization sites
from the audit. No behavior change.

Changes

• Collection expressions replacing the older initializer syntax:
  • Models/TargetPreset.cs — All = new[] { ... } → [ ... ]
  • Services/AppAlertService.cs — uninstall paths new[] { ... } → string[] paths = [ ... ]
  • Services/DeepCleanupService.cs — roots = new List<string> { ... } → List<string> roots = [ ... ] (still a mutable List; later .Add() preserved)
  • Services/DuplicateFileService.cs — new Dictionary<long, List<FileInfo>>() → Dictionary<long, List<FileInfo>> sizeGroups = []
  • Services/FileShredderService.cs — new Stack<DirectoryInfo>() → Stack<DirectoryInfo> stack = []
  • SysManager.Tests/EventLogServiceTests.cs — new List<EventSeverity> { ... } → [ ... ]
• Pattern matching: Helpers/EqualityConverter.cs — value == null || parameter == null
  → value is null || parameter is null.

Not converted (deliberately)

• Models/ContextMenuPreset.cs dictionary-with-indexer-initializer — collection expressions
  don't cleanly express keyed dictionary initializers, so it's left as new Dictionary<...>{ [k]=v }.

Verification

• Build: main + Tests 0 warnings / 0 errors (clean --no-incremental rebuild).
• Every change is byte-identical in behavior (same collection contents / same null check).

refactor: → no release.