refactor(config): consolidate settings into one grouped settings.json

App preferences were spread across six flat plain-text files (settings.txt,
recent.txt, recent_limit.txt, statusbar.txt, ribbon.txt, plot.txt), each with a
bespoke line parser and no grouping. Replace them with a single grouped JSON,
serialized via serde (enabling the derive feature; serde_json was already a
dependency).

- New app/config.rs: AppConfig { settings, recent, statusbar, ribbon, plot } →
  <config>/settings.json (pretty JSON), load()/save().
- Make UserSettings, StatusBarConfig/StatusPill, CollapseMode and
  PlotDialogState serde-derived; #[serde(skip)] marks the plot dialog's runtime
  fields so only print preferences persist. Their bespoke .txt load/save is
  removed.
- App gains current_config/apply_config/save_config: one AppConfig::load at
  startup distributes into live state; every settings-change site
  (persist_settings_if_changed, recent add/remove/limit, status-bar pill
  toggle, ribbon density, plot commit) routes through save_config, which diffs
  against the last write to avoid thrashing.
- Clean start: the old flat files are ignored (no migration). ocad.pgp stays a
  separate hand-editable file.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/update/mod.rs

@@ -641,6 +641,7 @@ impl OpenCADStudio {
             Message::SetRibbonCollapseMode(mode) => {
                 self.ribbon.set_collapse_mode(mode);
                 self.ribbon.close_dropdown();
+                self.save_config();
                 Task::none()
             }
 
@@ -1674,6 +1675,7 @@ impl OpenCADStudio {
             Message::ToggleStatusPill(pill) => {
                 // Keep the menu open so several pills can be toggled in a row.
                 self.statusbar_config.toggle(pill);
+                self.save_config();
                 Task::none()
             }
             Message::ToggleCleanScreen => {