docs: complete XML doc coverage (returns, summaries, inheritdoc)

Resolve all 622 issues flagged by the enhanced CommentChecker: add missing
<returns> tags (incl. the standard phrasing on non-generic Task methods),
add missing <summary> tags, and replace misused/redundant <inheritdoc/> on
members that override or implement nothing with real documentation.
Documentation-only — no behavior change; solution builds clean.

src/ZB.MOM.WW.ScadaBridge.CentralUI/Auth/ClaimsPrincipalExtensions.cs

@@ -20,6 +20,7 @@ public static class ClaimsPrincipalExtensions
     /// <see cref="UnknownUser"/> when the claim is absent.
     /// </summary>
     /// <param name="principal">The claims principal to read the username from.</param>
+    /// <returns>The username claim value, or <see cref="UnknownUser"/> if absent.</returns>
     public static string GetUsername(this ClaimsPrincipal principal)
         => principal.FindFirst(JwtTokenService.UsernameClaimType)?.Value ?? UnknownUser;
 
@@ -28,6 +29,7 @@ public static class ClaimsPrincipalExtensions
     /// the claim is absent.
     /// </summary>
     /// <param name="principal">The claims principal to read the display name from.</param>
+    /// <returns>The display name claim value, or <c>null</c> if the claim is absent.</returns>
     public static string? GetDisplayName(this ClaimsPrincipal principal)
         => principal.FindFirst(JwtTokenService.DisplayNameClaimType)?.Value;
 
@@ -37,6 +39,7 @@ public static class ClaimsPrincipalExtensions
     /// ten components (CentralUI-024).
     /// </summary>
     /// <param name="authStateProvider">The Blazor authentication state provider to read from.</param>
+    /// <returns>A task that resolves to the current user's audit username, or <see cref="UnknownUser"/> if not authenticated.</returns>
     public static async Task<string> GetCurrentUsernameAsync(
         this AuthenticationStateProvider authStateProvider)
     {