Make feature flag checks async end-to-end

[Copilot]

IFeatureService.IsFeatureEnabled was synchronously blocking on IFeatureManagerSnapshot.IsEnabledAsync via .GetAwaiter().GetResult(), risking thread-pool starvation and deadlocks under ASP.NET request contexts.

Changes

• IFeatureService / FeatureService: Replaced bool IsFeatureEnabled(string) with Task<bool> IsFeatureEnabledAsync(string), using await in the implementation
• ViewComponents (UploadViewComponent, TabbedNavigationViewComponent): Updated calls to await IsFeatureEnabledAsync(...) inside existing async InvokeAsync methods
• Razor Pages (Index, Completed, Denied, Review, Justification): Converted OnGet() → OnGetAsync() returning Task, awaiting feature flag checks

// Before
public bool IsFeatureEnabled(string featureName) =>
    _featureManager.IsEnabledAsync(featureName).GetAwaiter().GetResult();

// After
public async Task<bool> IsFeatureEnabledAsync(string featureName) =>
    await _featureManager.IsEnabledAsync(featureName);

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