refactor(audience-sample): centralise "Copied!" flash label in SampleAppUi.ButtonText.CopiedFlash

AudienceSample.UI.cs:368 set the click-to-copy flash via inline
"Copied!". Distinct from the existing SampleAppUi.ButtonText.Copied
("Copied", no exclamation) used as the persistent button label after
copying. Both strings can drift independently today.

Adds ButtonText.CopiedFlash with a comment noting the distinction
and migrates the inline reference. The two consts now sit next to
each other so a future translation pass would catch both.

Per the user's "everything random goes in a constant" stance.

Follow-up to SDK-272 (centralisation of duplicated literals).

Author: ImmutableJeffrey

examples/audience/Assets/SampleApp/Scripts/AudienceSample.UI.cs

@@ -365,7 +365,7 @@ private void RegisterHandlers()
                     if (label.ClassListContains(SampleAppUi.Css.Copied)) return;
                     if (string.IsNullOrEmpty(label.text) || label.text == SampleAppUi.StatusBar.EmptyText) return;
                     GUIUtility.systemCopyBuffer = label.text;
-                    label.text = "Copied!";
+                    label.text = SampleAppUi.ButtonText.CopiedFlash;
                     FlashCopied(label);
                 });
         }

examples/audience/Assets/SampleApp/Scripts/SampleAppUi.cs

@@ -160,6 +160,10 @@ internal static class ButtonText
             internal const string Send = "Send";
             internal const string Copy = "Copy";
             internal const string Copied = "Copied";
+
+            // Click-to-copy flash on status cells (transient label override).
+            // Distinct from Copied which is the post-click button label.
+            internal const string CopiedFlash = "Copied!";
         }
 
         // ---- Resources paths ----