Phase 3: Refactor ShellNavigationService to use DI

Scope:\n- Replace global container lookups in ShellNavigationService.\n- Use injected dependencies, factories, or IServiceProvider where appropriate.\n- Preserve current navigation behavior.\n\nAcceptance criteria:\n- ShellNavigationService no longer depends on MauiProgram.Container.\n- Navigation still resolves pages successfully.\n- No regressions in back-navigation or page instantiation.