Fix output suffix handling when loading presets

[LadaN62]

Fixes output filename handling when switching between presets with different suffixes.

A derived output filename could previously be interpreted as an explicitly selected filename after loading another preset. As a result, the output filename was no longer recomputed with the newly loaded suffix, and suffixes could accumulate, for example:

document_EM.pdf -> document_EM_DL.pdf

instead of:

document_EM.pdf -> document_DL.pdf

The change keeps automatically derived output names in derived mode when a preset is loaded, while preserving explicitly selected output filenames.

It also clears persisted output-file state when starting a new application session, so the first opened document does not inherit a stale output filename from the previous session.

Verification

• Opening a PDF starts with the expected derived output name.
• Loading a preset with _EM applies _EM.
• Switching to a preset with _DL replaces _EM with _DL instead of accumulating suffixes.
• Toggling "Append suffix to filename" updates the derived output filename.
• Explicitly entered output filenames remain preserved.
• mvn -pl distribution -am -DskipTests clean package: BUILD SUCCESS.
• mvn clean install: 5 DSS test failures. The same 5 failures reproduce unchanged on a clean, unmodified upstream/master checkout at commit 6c9623d.

Documentation

No documentation change is needed. website/docs/JSignPdf.adoc already describes the intended behavior: the output suffix is stored in presets and is used to derive the output filename unless an explicit output file is selected.