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>

Cargo.toml

@@ -22,8 +22,10 @@ default = ["solid3d"]
 solid3d = ["dep:truck-meshalgo", "dep:truck-shapeops", "dep:lzma-sys"]
 
 [dependencies]
-# JSON: plugin marketplace / Patreon fetch (native) + web supporters.json parse.
+# JSON: plugin marketplace / Patreon fetch (native) + web supporters.json parse,
+# and the consolidated user config (settings.json) via serde-derived structs.
 serde_json = "1"
+serde = { version = "1", features = ["derive"] }
 # Stable, dependency-free add-on contract (manifest + ribbon/CadModule types).
 # Plugin authors target this crate's semver, not OpenCADStudio internals.
 # The `host` feature (out-of-process plugin runtime: interprocess, libloading,