Add a Feature Without Breaking the Conventions

Background: Existing native [iOS / Android] codebase using [architecture]
with [DI approach], [networking layer], and [persistence]. Here are two
representative existing features so you can see how we do things:

[Paste one view model + its repository/use case, and the corresponding
view]

Use case: I need to add [describe the feature]. Write it so a reviewer
can't tell it was added later.

Implementation details:
- Match the existing conventions: naming, file layout, error handling,
  threading, and how state is exposed to the view
- Reuse what already exists rather than introducing a parallel mechanism—
  if you're tempted to add a new abstraction, justify it explicitly
- Follow the same testing approach the existing code uses
- Note anywhere the existing convention is actively bad and adding to it
  makes things worse, but keep that separate from the implementation

Deliverable: The complete implementation in the codebase's own idiom, its
tests written the way the existing tests are written, a note of every
existing file that needs touching, and a short "conventions I followed"
list so I can check you read the codebase rather than a tutorial.