fix: handle empty CSV data and empty collection on startup

[nazifsco]

Fixes two RangeError crashes:

Fix 1 -- CsvPreviewer crashes on empty CSV response (closes #1647)

lib/widgets/previewer_csv.dart accessed csvData[0] without checking if the list was empty. Any API response returning empty or malformed CSV would crash the app.

// before
final List<List<dynamic>> csvData = csv.decode(body);
return SingleChildScrollView(...) // crashes if csvData is empty

// after
final List<List<dynamic>> csvData = csv.decode(body);
if (csvData.isEmpty) return errorWidget;
return SingleChildScrollView(...)

Fix 2 -- CollectionStateNotifier crashes on empty collection (closes #1640)

lib/providers/collection_providers.dart accessed requestSequenceProvider[0] unconditionally. On first launch or after clearing all requests, this crashed with RangeError: Index out of range.

// before
ref.read(selectedIdStateProvider.notifier).state = ref.read(
  requestSequenceProvider,
)[0];

// after
final ids = ref.read(requestSequenceProvider);
ref.read(selectedIdStateProvider.notifier).state =
    ids.isNotEmpty ? ids[0] : null;