fix(file-list): align Previous and Next with visible Images

This was generated by AI.

Problem

Previous and Next are enabled without regard to the current File List position. At a boundary their handlers only log and return, so enabled actions can do nothing. In-memory filtering also needs one explicit navigation contract.

Required behavior

• Previous and Next navigate only among Images visible under the active File List filter.
• If the active Image is hidden by the filter, Previous selects the nearest visible Image before it in the original directory order, and Next selects the nearest visible Image after it.
• Disable an action when no visible Image exists in that direction.
• Keep both actions disabled for an empty visible result set.
• Recompute action state after import, filtering, successful navigation, and clearing the filter.
• Apply the same behavior to menu, toolbar, and keyboard shortcut activation.

This builds on #2455, which changes search to an in-memory filter without navigating or replacing the active session.

Verification

Add end-to-end cases for the first and last visible Images, one visible Image, no visible Images, a hidden active Image with matches on both sides, and clearing the filter.