Persist selected filter on Upcoming screen #142

Description

The Upcoming screen will introduce pill-shaped filter dropdowns for completion status as part of #139.
The available filters are: All, Unfinished, and Completed. The default selected filter is "Unfinished".
Currently the filter value is held only in memory inside the ViewModel. When the user leaves the screen, kills the app, or the process is recreated, the filter resets to "Unfinished". We need to persist the last selected filter so it is restored on the next app launch.

• Scope and constraints

  • Must work across Android, iOS, and Desktop.
  • Must reuse the project's existing persistence stack (Koin + Room) where possible.
  • Depends on Upcoming screen: day-grouped task list #138, Upcoming screen: pill-shaped filter dropdowns #139, Sort by date (due date, creation date) #140, and Sort by priority #141.

• References

  • Upcoming screen: day-grouped task list #138
  • Upcoming screen: pill-shaped filter dropdowns #139
  • Sort by date (due date, creation date) #140
  • Sort by priority #141

Scope

In scope: a persistence layer for the active completion-status filter value (All, Unfinished, Completed), read/write from the UpcomingViewModel, and restoration on startup.

Out of scope: syncing the preference across devices or to Supabase.

Acceptance criteria

• GIVEN the user changes the filter on the Upcoming screen
  WHEN the app process is killed and restarted
  THEN the previously selected filter (All, Unfinished, or Completed) is restored
• GIVEN the filter value is persisted
  WHEN the app reads it on launch
  THEN the default filter "Unfinished" is used if no prior value was saved
• GIVEN the filter preference is stored locally
  WHEN unit tests run
  THEN the storage layer has test coverage with Kotest assertions