Add named signature-setting presets (+ XDG-aware config location)

[kwart]

Adds named signature-setting presets to the JavaFX UI and moves per-user config to a platform-native location.

Closes #252 (config-file location). Presets had no dedicated issue — requested by a user who regularly switches between several signing configurations.

What's new

• Presets — save the current signing configuration under a name, switch between them with one click.
  • Toolbar drop-down: picking an entry loads the preset immediately (fire-and-forget — no "active preset" tracked).
  • Presets menu: Save current as new preset… and Manage presets… (rename / overwrite / delete, with confirmations for the destructive actions).
  • Presets capture the whole signing configuration except session state (input/output paths).
• Config directory — moves per-user state out of ~/.JSignPdf into:
  • Linux/BSD: $XDG_CONFIG_HOME/jsignpdf/ (fallback ~/.config/jsignpdf/)
  • Windows: %APPDATA%\JSignPdf\
  • macOS: ~/Library/Application Support/JSignPdf/
  • Override with JSIGNPDF_CONFIG_DIR env var.
  • One-shot migration: on first run, if the new dir is absent and ~/.JSignPdf exists, it's copied to <cfg>/config.properties. The legacy file is left in place so downgrades keep working.

Internals

• PropertyProvider lost its singleton; PropertyStoreFactory hands out the cached main-config store and fresh preset stores (preset-<epoch-ms>.properties with collision retry).
• BasicSignerOptions got loadFromPreset / storeToPreset alongside the existing loadOptions / storeOptions, gated by a shared flag so the main-config path and preset path don't drift.
• VisibleSignatureCoordinator centralises placement ↔ signing-VM sync (previously duplicated inline in two controller methods) so the visible-signature rectangle survives the save → load round-trip.
• Reset Settings now wipes the whole config directory (including presets/), exposed as a testable PropertyStoreFactory.resetAll().

Docs

• Design doc: design-doc/3.0.0-presets-feature.md.
• User guide: new Using presets section in the JavaFX chapter and a Configuration directory section under Advanced configuration. Rebuilt the asciidoctor PDF locally as a smoke check.

Test plan

• mvn test — 160/160 pass (106 existing + 54 new across ConfigLocationResolver, PropertyStoreFactoryReset, PresetManager, PresetValidation, VisibleSignatureCoordinator).
• asciidoctor PDF builds without errors.
• Manual UI smoke: save preset → restart → load → verify settings + visible-signature rectangle reappear.
• Manual UI smoke: Manage dialog rename / overwrite / delete reflect in the toolbar combo without restart.
• Fresh Linux install (no ~/.JSignPdf): new config dir created, combo shows No presets saved.
• Upgrade path: existing ~/.JSignPdf → new <cfg>/config.properties appears, legacy file untouched.