feat(profile): add local backup export/import UI (#84 F1/F2)

- F1: "Sauvegarde locale" panel in Profil (both signed-out/signed-in
  states, positioned per UX cadrage), "Exporter mes données" calling
  DataExportUseCase.exportAll() then handing the file to the system
  share sheet, with the exact UX success/error snackbars.
- F2: "Importer des données" picks a file, previews it, then confirms
  with the exact UX dialogs: simplified single-choice when there is no
  local data yet, Fusionner/Remplacer tout (with a destructive second
  confirmation) otherwise. All typed LocalBackupValidationError cases
  are mapped to their dedicated dialog copy, including the active
  workout session block.

lib/presentation/home_screen.dart

@@ -227,6 +227,8 @@ final class _HomeScreenState extends State<HomeScreen> with RouteAware {
                   authUseCases: widget.bootstrap.authUseCases,
                   syncUseCases: widget.bootstrap.syncUseCases,
                   shareUseCases: widget.bootstrap.shareUseCases,
+                  dataExportUseCase: widget.bootstrap.dataExportUseCase,
+                  dataImportUseCase: widget.bootstrap.dataImportUseCase,
                 ),
               ),
             ),