fix: normalize style with full defaults when chart enters Redux

All style sub-objects (title, legend, xAxis, yAxis, dataLabel,
background, border, animation) are guaranteed to exist in Redux.
This fixes:
- Style config panels showing blank (returned null for missing fields)
- Style changes not applying (updateStyle now deep-merges sub-objects)
- Color palette changes not persisting

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/adapters/state/redux/chartSlice.ts

@@ -4,6 +4,33 @@ import { type FieldBinding } from '@/domain/entities/FieldBinding';
 import { type StyleConfig } from '@/domain/entities/StyleConfig';
 import { type SortConfig } from '@/domain/valueObjects/SortOrder';
 
+const DEFAULT_STYLE: StyleConfig = {
+  title: { text: '', visible: false, fontSize: 16, fontWeight: 'bold', color: '#333', align: 'left' },
+  colors: ['#5470c6', '#91cc75', '#fac858', '#ee6666', '#73c0de', '#3ba272', '#fc8452', '#9a60b4'],
+  legend: { visible: true, position: 'top', orient: 'horizontal', fontSize: 12, color: '#666' },
+  xAxis: { visible: true, labelVisible: true, labelFontSize: 12, labelColor: '#666', labelRotation: 0, titleVisible: false, titleText: '', gridVisible: true, gridColor: '#eee' },
+  yAxis: { visible: true, labelVisible: true, labelFontSize: 12, labelColor: '#666', labelRotation: 0, titleVisible: false, titleText: '', gridVisible: true, gridColor: '#eee' },
+  dataLabel: { visible: false, fontSize: 12, color: '#333', position: 'outside', format: 'value' },
+  background: { color: 'transparent', opacity: 1, borderRadius: 4 },
+  border: { visible: false, color: '#ddd', width: 1, style: 'solid' },
+  animation: { enabled: true, duration: 500, easing: 'ease-out' },
+};
+
+function ensureStyle(style: any): StyleConfig {
+  if (!style || typeof style !== 'object') return { ...DEFAULT_STYLE };
+  return {
+    title: { ...DEFAULT_STYLE.title, ...(style.title ?? {}) },
+    colors: (Array.isArray(style.colors) && style.colors.length > 0) ? style.colors : DEFAULT_STYLE.colors,
+    legend: { ...DEFAULT_STYLE.legend, ...(style.legend ?? {}) },
+    xAxis: { ...DEFAULT_STYLE.xAxis, ...(style.xAxis ?? {}) },
+    yAxis: { ...DEFAULT_STYLE.yAxis, ...(style.yAxis ?? {}) },
+    dataLabel: { ...DEFAULT_STYLE.dataLabel, ...(style.dataLabel ?? {}) },
+    background: { ...DEFAULT_STYLE.background, ...(style.background ?? {}) },
+    border: { ...DEFAULT_STYLE.border, ...(style.border ?? {}) },
+    animation: { ...DEFAULT_STYLE.animation, ...(style.animation ?? {}) },
+  };
+}
+
 interface ChartState {
   charts: ChartInstance[];
   activeChartId: string | null;
@@ -19,7 +46,8 @@ const chartSlice = createSlice({
   initialState,
   reducers: {
     addChart(state, action: PayloadAction<ChartInstance>) {
-      state.charts.push(action.payload);
+      const chart = { ...action.payload, style: ensureStyle(action.payload.style) };
+      state.charts.push(chart);
     },
     removeChart(state, action: PayloadAction<string>) {
       state.charts = state.charts.filter((c) => c.id !== action.payload);
@@ -46,7 +74,21 @@ const chartSlice = createSlice({
     ) {
       const chart = state.charts.find((c) => c.id === action.payload.chartId);
       if (chart) {
-        chart.style = { ...chart.style, ...action.payload.style };
+        // Deep merge each sub-object
+        const incoming = action.payload.style;
+        const current = chart.style as StyleConfig;
+        chart.style = {
+          ...current,
+          ...incoming,
+          title: incoming.title ? { ...current.title, ...incoming.title } : current.title,
+          legend: incoming.legend ? { ...current.legend, ...incoming.legend } : current.legend,
+          xAxis: incoming.xAxis ? { ...current.xAxis, ...incoming.xAxis } : current.xAxis,
+          yAxis: incoming.yAxis ? { ...current.yAxis, ...incoming.yAxis } : current.yAxis,
+          dataLabel: incoming.dataLabel ? { ...current.dataLabel, ...incoming.dataLabel } : current.dataLabel,
+          background: incoming.background ? { ...current.background, ...incoming.background } : current.background,
+          border: incoming.border ? { ...current.border, ...incoming.border } : current.border,
+          animation: incoming.animation ? { ...current.animation, ...incoming.animation } : current.animation,
+        };
         chart.updatedAt = new Date().toISOString();
       }
     },