Store and show groups offline

[f-odc]

Now users can see their groups and balance even if they are offline.
This PR was created in anticipation of longer server downtime or a permanent shutdown.

Closes #296

[Copilot]

Pull Request Overview

This PR implements offline storage functionality that allows users to view their groups and balance even when the server is unavailable, providing resilience against potential server downtime or shutdown.

• Adds offline storage methods for groups, bills, and items using SharedPreferences
• Implements fallback logic to load cached data when server requests fail
• Enhances existing domain models with offline-specific serialization methods

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
lib/infrastructure/shared_preferences.dart	Adds methods to store and retrieve groups from local storage
lib/domain/group/states/groups_state.dart	Implements try-catch logic to fallback to cached data when server is unavailable
lib/domain/group/group.dart	Adds toMapOffline() method for complete group serialization
lib/domain/bill/item.dart	Adds toMapOffline() method for item serialization
lib/domain/bill/bill.dart	Adds toMapOffline() method for bill serialization

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[GR0ZA]

nice work 🚀
just one small thing

[GR0ZA]

the comments are kinda superfluous here 😅 it's pretty clear what's happening already

[GR0ZA]

IMO this caching logic should live inside the repository instead of here. The repository is responsible for data access, whether it comes from the network or local storage (yeah I know maybe we've done it differently somewhere else). This way we don't have to think about loading the local storage when we want to fetch groups, the repository does it automatically for us.

The repository could receive SharedUtility as a dependency:

@Riverpod(keepAlive: true)
GroupRepository groupRepository(Ref ref) {
  return RemoteGroupRepository(
    api: GroupAPI(),
    client: ref.read(httpClientProvider),
    sharedUtility: ref.read(sharedUtilityProvider),
  );
}

And the repository method could look like this:

...
final SharedUtility sharedUtility;
...

@override
Future<List<Group>> getGroupsByUser(String userId) async {
  try {
    final groups = await client.get(
      uri: api.getGroupsByUser(userId),
      builder: (data) {
        if (data == null || data.isEmpty) return [];
        return data.map((g) => Group.fromMap(g)).toList();
      },
    );

    sharedUtility.setGroups(groups);
    return groups;
  } catch (e) {
    return sharedUtility.getGroups();
  }
}

[f-odc]

Changed!