Mark as completed / undo on Task Detail screen #135

Description

Add a primary action on the Task Detail screen that allows the user to mark the task as completed or undo the completion. When the user confirms the action, the app updates completedAt via the task repository. The UI should reactively reflect the new completion state.

This provides an explicit, screen-level control for completion so users can mark a task done even when they opened it for reading rather than from a list row.

Scope

In scope: a button or toggle action on the Task Detail screen that sets or clears completedAt, reactive state update, error handling, and string resources.
Out of scope: swipe gestures (separate ticket), Snackbar undo on the detail screen, bulk actions, cloud sync logic.

Acceptance criteria

• GIVEN the user is on the Task Detail screen for an unfinished task
  WHEN they tap the "Mark as completed" action
  THEN the task is marked as done and completedAt is set to the current instant
• GIVEN the user is on the Task Detail screen for a completed task
  WHEN they tap the "Mark as not completed" action
  THEN the task is marked as unfinished and completedAt is cleared
• GIVEN the task's completion status changes from the detail screen
  WHEN the repository updates
  THEN the detail screen reflects the new state reactively without requiring a manual refresh
• GIVEN user-visible labels or button text appear on the detail screen
  WHEN the app renders them
  THEN all visible text comes from string resources