Use file picker for import on Android instead of showing manual instructions

[Copilot]

The import data button in settings showed a message box with manual file opening instructions on Android, while using a native file picker on Windows.

Changes

• Removed platform check in PickAndStartImport method in SettingsPage.razor
• Android now uses FilePicker.PickAsync() like other platforms

// Before: Platform-specific behavior
if (DeviceInfo.Current.Platform == DevicePlatform.WinUI)
{
    var pickResult = await FilePicker.PickAsync();
    // ... import logic
}
else
{
    await DialogService.ShowJaMessageBox("Find the backup in your files app...");
}

// After: Consistent file picker across all platforms
var pickResult = await FilePicker.PickAsync();
if (pickResult == null)
    return;
App.ActivatedFilePath = pickResult.FullPath;
await StartImport();

MAUI's FilePicker supports Android natively, so the platform check was unnecessary.

Original prompt

Make the import data button in settings open a file picker on android instead of just showing a message box with insrtuctions on how to open the file manually

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