refactor: drop two orphan DI registrations for static-only services

[laurentiu021]

Summary

Removes two dead DI registrations. IconExtractorService and ServiceManagerService
expose only static members and every call site is a static call — the container
constructed an unused empty singleton of each at startup that nothing ever resolved.
Deleting the registrations removes that dead wiring without touching any call site.

Changes

ServiceRegistration.cs:

• Removed services.AddSingleton<IconExtractorService>();
• Removed services.AddSingleton<ServiceManagerService>();

Both classes remain exactly as-is (static utility classes); all static call sites are
untouched.

Why this is safe / behavior-identical

• Verified neither type is ever resolved from the container
  (GetService/GetRequiredService) or constructor-injected anywhere — both are
  pure static-method utilities. The build proves there is no compile-time dependency.
• No test (unit or integration) asserts these registrations exist.
• The only runtime effect was the container eagerly new-ing one unused instance of
  each on first build of the provider; nothing read those instances, so removing them
  changes no observable behavior.

Note: AppBlockerService — flagged alongside these in the original audit — is not
an orphan anymore; it was converted to an injected instance behind IAppBlockerService
in an earlier PR and is correctly registered. Left untouched.

Verification

• Build: main + Tests + IntegrationTests all 0 warnings / 0 errors.

Behavior identical (Protocol Q). refactor: → no release.