Change the default values in the new UI

[kwart]

Summary

• Append signature and Store passwords checkboxes in the JavaFX UI are now checked by default on fresh installs. Previously both defaulted to unchecked, despite DEFVAL_APPEND=true existing in Constants.
• Root cause: BasicSignerOptions.loadOptions() called getAsBool(key) without a default, so missing keys silently resolved to false and overrode the field's initial value. Fixed by passing DEFVAL_APPEND / new DEFVAL_STOREPWD as fallbacks.
• CLI behavior is unaffected: the -a flag always overrides append, and storePasswords is not used by the CLI flows.
• Users with existing ~/.JSignPdf preferences keep their saved values — both keys are always persisted by storeOptions(), so only truly fresh installs (or post "Reset Settings") see the new defaults.

Test plan

• New unit tests in BasicSignerOptionsTest: default field values, loadOptions() with empty props, explicit-false preserved.
• SigningOptionsViewModelTest: new fresh-VM defaults test; updated resetToDefaults expectation for storePasswords.
• mvn -pl jsignpdf test — 110 tests pass.
• Manual smoke check: launch JavaFX UI with no ~/.JSignPdf; verify both checkboxes start checked.
• Manual smoke check: uncheck both, close, relaunch; verify the unchecked state is restored.

Docs

• website/docs/JSignPdf.adoc — Append signature and Remember passwords sections updated.
• distribution/doc/release-notes/3.0.0.md — bullet added.