SettingsScreen: Added import button that opens the system file picker
(ActivityResultContracts.OpenDocument) filtered to application/json.
After file selection, a confirmation dialog warns that existing data
will be overwritten. Import result is shown in a success/error dialog.
SettingsViewModel: Added onImportFileSelected(uri), onImportConfirmed(),
onImportDismissed(), onImportResultDismissed() methods. The import reads
the file via contentResolver.openInputStream() and delegates to the
existing ImportExportRepository.importFromJson(). Settings are reloaded
after successful import.
SettingsUiState: Extended with isImporting, importResult (sealed
interface ImportResult with Success/Error), and pendingImportUri for
the confirmation dialog flow.
SettingsViewModelTest: Added 6 unit tests covering import success,
invalid JSON error, empty file, null input stream, dialog state
management, and result dismissal.
Closes#38
ce851af37b