refactor(apple): remove diagnostics opt-in toggle, keep bug reports

Drop the Share-diagnostics preference, its Settings toggle and the
DiagnosticsBuildConfig stub. Bug reporting (NoopBugReportService) and the
diagnostics install id used for bug-report correlation are retained.

apple/VniDrop/App/AppGraph.swift

@@ -24,8 +24,7 @@ final class AppGraph: ObservableObject {
 			fallback: AppPreferencesDefaults(
 				username: dependencies.environment.defaultUsername,
 				receiveFolder: dependencies.fileSystemService.defaultReceiveFolder(),
-				themeMode: .system,
-				diagnosticsEnabled: false
+				themeMode: .system
 			)
 		)
 		self.approvalCoordinator = ApprovalCoordinator(