Fix orphaned loop detection progress dialog

[tandede]

Summary

Fixes #1462.

DatabaseViewer::detectMoreLoopClosures() currently constructs and shows its progress dialog before validating the session-selection options. When both intra-session and inter-session detection are disabled and no graph nodes are selected, the function displays the validation warning and returns, leaving the already-visible progress dialog behind.

Implementation

Move the existing progress-dialog construction block below that validation guard. No dialog settings or loop-closure behavior are changed:

• invalid configurations now return after the warning without creating a progress dialog;
• valid configurations create the same dialog with the same ownership, cancellation, sizing, and completion behavior as before.

I kept this as a pure control-flow fix rather than adding cleanup to the error branch, so the resource is never created when no work can start.

Validation

• confirmed the issue path on current master: progress-dialog construction preceded the invalid-option guard;
• confirmed the updated path evaluates the guard before constructing the dialog;
• verified the relocated seven-line construction block is byte-for-byte unchanged;
• git diff --check passes;
• the change is limited to guilib/src/DatabaseViewer.cpp (+7/-7).

The repository does not currently provide a GUI test target for DatabaseViewer; the existing cross-platform CI builds will cover compilation of the relocated code.

[matlabbe]

Looks good to me! will merge after CI run.