refactor(engine): split settings_plugin runtime code into submodules

Second phase of #118 for settings_plugin: the 2,857-line mod.rs becomes
four focused submodules along existing system boundaries —

  mod.rs     561  types, markers, SettingsButton, plugin build, persistence
  input.rs   632  button/hotkey handlers, focus attachment, scrolling
  updates.rs 495  per-frame value-text updater systems + label helpers
  ui.rs    1,231  spawn_settings_panel + row builders + thumbnails

Moved items are pub(super); mod.rs glob-imports the submodules so
system registration and tests keep their bare names. No behaviour
change; all 29 settings tests pass unchanged.

Refs #118

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/settings_plugin/tests.rs

@@ -1,4 +1,7 @@
 use super::*;
+use crate::ui_focus::{FocusRow, Focusable};
+use crate::ui_modal::ModalButton;
+use crate::ui_tooltip::Tooltip;
 
 fn headless_app() -> App {
     let mut app = App::new();
@@ -326,9 +329,7 @@ fn settings_buttons_carry_tooltip() {
         .world_mut()
         .query::<(&SettingsButton, &Tooltip)>()
         .iter(app.world())
-        .find_map(|(btn, tip)| {
-            matches!(btn, SettingsButton::ConnectSync).then(|| tip.0.clone())
-        })
+        .find_map(|(btn, tip)| matches!(btn, SettingsButton::ConnectSync).then(|| tip.0.clone()))
         .expect("Connect button should spawn with a Tooltip when backend is Local");
     assert_eq!(
         connect_tip.as_ref(),