refactor: cycle 18 UI - remove dead dependencies and dead code

[rygel]

Cycle 18 Lean Code Sweep (UI tasks).

Changes:

• Remove dead System.Reactive NuGet from UI.Slim and Tests (~2MB savings)
• Delete dead methods ShouldRenderAsSettingsSubItem and GetEligibleSubTrayDetails + 2 test files
• Delete dead GetIsCollapsed/SetIsCollapsed stubs + test file
• Delete unused HttpHeaders.cs constants file
• Remove ~30 unused constants from ProviderEndpoints.cs (140 to 23 lines)

All changes verified with dotnet build + dotnet test (1353 passed, 0 new failures).