Add fixed 'Completed' column to Kanban

Added

• Automatic 'Completed' column on the Makerkit Kanban board.
• Users can still create as many custom columns and tasks as they want.
• Any task moved into the 'Completed' column is treated as finished and used for completion metrics.

Behavior

• The 'Completed' column is fixed: it cannot be renamed, deleted, or modified by users.
• This ensures a single, consistent place where all done tasks live across all boards.
• Future analytics and progress tracking can reliably use this column to calculate completed work.

Benefits

• Clear definition of done: every board uses the same canonical column for finished tasks.
• Consistent reporting: makes it easier to track completed tasks over time and across projects.
• Flexible workflow: teams keep full freedom to design their own process with custom columns, while still having a standardized end state for tasks.