Add attachments to events and exams

[nilsreichardt]

Motivation

• Enable attaching files to calendrical events/exams so users can upload information sheets or exam summaries like they already can for homework and blackboard items.
• Reuse the existing filesharing flows to store attachments and keep reference tracking in Firestore so files are associated with the created/edited event.

Description

• Add attachments: List<String> to CalendricalEvent and include it in serialization/deserialization and copyWith so events can store attachment IDs.
• Extend the filesharing reference types with a new ReferenceType.event so files can be referenced by events.
• On event creation upload local files via FileSharingGateway.uploadAttachments, persist returned file IDs on the event, and add reference data to each file; TimetableGateway.createEvent now returns the created eventID needed for reference tracking.
• On event edit support adding new local files (upload + add reference data) and removing existing cloud files (remove reference data); the edit flow builds the final attachments list and saves it.
• Add UI components to the event add dialog and the event edit page using the shared AttachFile/AttachFileBase widgets so users can add/remove attachments when creating or editing events.
• Show attachments on the event details page using AttachmentStreamList, and remove file references when an event is deleted.
• Update mocks and tests to account for new return types and the new attachments field.

Testing

• Ran fvm flutter test app/test/timetable/timetable_dialog_test.dart and the test suite passed.
• Ran fvm flutter test app/test/timetable/timetable_bloc_test.dart and the test suite passed.

---

Codex Task

[gemini-code-assist]

Code Review

This pull request successfully adds the functionality to attach files to calendrical events and exams. The changes are well-integrated, reusing existing filesharing flows and updating models, UI components, and gateways accordingly. The logic for creating, editing, and deleting events now correctly handles attachment references. I've found one area for improvement in TimetableEditEventBloc to make the code more concise and idiomatic, which I've detailed in a specific comment.

[gemini-code-assist]

These methods for updating file lists can be made more concise and idiomatic by using modern Dart collection features like the spread operator to create new lists. This improves readability and aligns with the style guide's recommendation to use collection literals where possible.

  Function(List<LocalFile>) get addLocalFiles => (localFiles) {
    _localFilesSubject.sink.add([..._localFilesSubject.value, ...localFiles]);
  };
  Function(LocalFile) get removeLocalFile => (localFile) {
    final newList = [..._localFilesSubject.value]..remove(localFile);
    _localFilesSubject.sink.add(newList);
  };
  Function(CloudFile) get removeCloudFile => (cloudFile) {
    final newList = [..._cloudFilesSubject.value]..remove(cloudFile);
    _cloudFilesSubject.sink.add(newList);
  };

References

1. The style guide recommends using collection literals when possible (line 86). The suggested change uses list literals with the spread operator for a more concise implementation. ^(link)

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