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/ui/ribbon/mod.rs

@@ -161,15 +161,20 @@ impl Ribbon {
             active_table_style: String::new(),
             tab_bar_h: Arc::new(AtomicU32::new(28.0f32.to_bits())),
             tool_bar_h: Arc::new(AtomicU32::new(TOOL_BAR_H.to_bits())),
-            collapse_mode: CollapseMode::load(),
+            collapse_mode: CollapseMode::default(),
             collapse_tight: Arc::new(AtomicBool::new(false)),
         }
     }
 
-    /// Change the tool-panel density and persist the choice.
+    /// Change the tool-panel density. Persistence is handled by the caller via
+    /// the consolidated app config (`save_config`).
     pub fn set_collapse_mode(&mut self, mode: CollapseMode) {
         self.collapse_mode = mode;
-        mode.save();
+    }
+
+    /// The current tool-panel density (for saving into the app config).
+    pub fn collapse_mode(&self) -> CollapseMode {
+        self.collapse_mode
     }
 
     /// Current tab-bar height as last measured by the `WrapBar` widget.