Swipe right to mark task as done #136

Description

Enable a swipe-right gesture on the task row component to quickly mark a task as done without tapping the checkbox. This gives users a fast, one-handed interaction for triaging tasks in list views such as Today and Upcoming.

After the swipe completes, the UI updates reactively to reflect the completed state.

Scope

In scope: swipe-right gesture on the task row component, updating completedAt in the repository, reactive UI updates, visual feedback during the swipe, and string resources.
Out of scope: swipe-left actions (e.g., delete or edit), undo Snackbar on list screens, bulk swipe, drag & drop.

Acceptance criteria

• GIVEN the user sees a task row in a list screen
  WHEN they swipe the row to the right
  THEN the task is marked as done and completedAt is set to the current instant
• GIVEN the user swipes a task row to mark it as done
  WHEN the repository updates
  THEN the hosting screen reflects the change reactively
• GIVEN the swipe gesture is active
  WHEN the row is being dragged
  THEN a visual indicator (e.g., icon or background color) communicates that the action will mark the task as done
• GIVEN user-visible text appears in the swipe indicator
  WHEN the app renders it
  THEN all visible text comes from string resources