table: Fix drag indicator side when moving column right (#2562)

When dragging a column header to the right, the column was dropped one
position past the indicated position, and the indicator jumped a whole
column at a time instead of tracking the actual insertion gap under the
cursor.

## Before


https://github.com/user-attachments/assets/07e9c334-fec9-4873-9d6e-d0019e7c90f8


## After


https://github.com/user-attachments/assets/a62f895e-c351-4e96-a13a-4a74df3ec1f8

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>