lighttable: preserve custom sort order when switching to custom sort

[engrzani]

Description

This PR fixes issue #12612 by preserving the current sort order when switching to "custom sort" mode.

Problem

Previously, when users selected "custom sort" from the sorting dropdown, the image order would reset to filename order. This made it impossible to:

• Sort by capture time first, then make minor adjustments
• Use any other sort criteria as a starting point for custom ordering

Solution

When switching to custom sort, the current sort order is now captured and stored in the position field, so users can make adjustments to the existing order rather than starting from scratch.

Implementation

• Added dt_collection_sync_custom_order() function to capture current order
• Hook in sort combobox change handler to call sync when switching TO custom sort
• Uses database transaction to ensure atomic updates
• Supports both regular and tagged collections

Testing

• Tested switching from various sort modes (filename, capture time, etc.) to custom sort
• Verified order is preserved correctly
• Tested with both regular collections and tagged collections
• No regression in existing custom sort functionality

Fixes #12612

[dterrahe]

Did you read any of the comments on #20162?