Add Phase button restored on phases overview

The Add Phase button is now accessible on the phases overview after a recent refactor temporarily hid the creation trigger.

Bug Fixes

The Add Phase button is now visible and functional on the phases overview page. A recent refactor moved the phase creation state into the client component but forgot to restore the UI trigger to open the phase creation sheet, making phase creation unreachable.

The button is now rendered in the section header action slot and remains visible at all times—whether the phase list is empty or populated. It is permission-gated, appearing only to users with phase creation access.