Chart theming: custom named themes + Theme Builder

src/app/stores/AppStore.ts

@@ -1,7 +1,7 @@
 import { create } from 'zustand';
 import type { FitMode } from '@core/rendering';
 import type { UiTheme } from '@core/theme';
-import type { ChartThemeId } from '@core/vega-themes';
+import type { ChartThemeSelection } from '@core/vega-themes';
 import type { ModalName } from '../modals/types';
 
 /**
@@ -25,7 +25,7 @@ export interface AppState {
   /** Preview sizing mode (spec §04); persisted to Settings as `previewFitMode`. */
   previewFitMode: FitMode;
   /** Chart theme selection (spec §04); persisted to Settings as `chartTheme`. */
-  chartTheme: ChartThemeId;
+  chartTheme: ChartThemeSelection;
   /** The currently open modal, or null. */
   activeModal: ModalName | null;
 
@@ -35,7 +35,7 @@ export interface AppState {
   /** Set the preview fit mode — the Live Preview Fit control's action. */
   setPreviewFitMode: (mode: FitMode) => void;
   /** Set the chart theme — the Live Preview settings cluster's action. */
-  setChartTheme: (theme: ChartThemeId) => void;
+  setChartTheme: (theme: ChartThemeSelection) => void;
   /**
    * Low-level modal setter — the single primitive that mutates `activeModal`.
    * High-level open/close (snapshot for unsaved-change detection, URL sync,