feat: add theme-aware chart colors for dark/light mode support

Replace hardcoded dark-theme hex colors in recharts components
(tooltips, axes) with a useChartTheme hook that reads the current
theme from ThemeContext. Charts now render correctly in both light
and dark mode.

Closes leeworks-agents/SPARC#1324

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

frontend/src/context/useChartTheme.ts

@@ -0,0 +1,41 @@
+import { useTheme } from './ThemeContext';
+
+/**
+ * Returns theme-aware color values for recharts components.
+ *
+ * Recharts accepts only raw color strings (not CSS variables),
+ * so this hook bridges the Tailwind/CSS-variable theme system
+ * to the imperative recharts API.
+ */
+export function useChartTheme() {
+  const { theme } = useTheme();
+  const isDark = theme === 'dark';
+
+  return {
+    /** Axis tick and grid line stroke color */
+    axisStroke: isDark ? '#94a3b8' : '#64748b',
+    /** Tooltip container background */
+    tooltipBg: isDark ? '#1e293b' : '#ffffff',
+    /** Tooltip container border */
+    tooltipBorder: isDark
+      ? '1px solid rgba(99, 102, 241, 0.3)'
+      : '1px solid rgba(99, 102, 241, 0.2)',
+    /** Tooltip label text color */
+    tooltipLabelColor: isDark ? '#f8fafc' : '#0f172a',
+    /** Tooltip item text color */
+    tooltipItemColor: isDark ? '#e2e8f0' : '#334155',
+    /** Convenience: full contentStyle object for recharts Tooltip */
+    tooltipContentStyle: {
+      backgroundColor: isDark ? '#1e293b' : '#ffffff',
+      border: isDark
+        ? '1px solid rgba(99, 102, 241, 0.3)'
+        : '1px solid rgba(99, 102, 241, 0.2)',
+      borderRadius: '8px',
+      color: isDark ? '#f8fafc' : '#0f172a',
+    },
+    /** Convenience: labelStyle for recharts Tooltip */
+    tooltipLabelStyle: {
+      color: isDark ? '#f8fafc' : '#0f172a',
+    },
+  };
+}

frontend/src/pages/Analytics.tsx

@@ -3,11 +3,13 @@ import { useQuery } from '@tanstack/react-query';
 import { analyticsApi } from '../api/client';
 import { AlertCircle, Database } from 'lucide-react';
 import { PieChart, Pie, Cell, BarChart, Bar, LineChart, Line, XAxis, YAxis, Tooltip, ResponsiveContainer, Legend } from 'recharts';
+import { useChartTheme } from '../context/useChartTheme';
 
 const COLORS = ['#6366f1', '#0ea5e9', '#10b981', '#f59e0b', '#ef4444', '#8b5cf6', '#ec4899', '#14b8a6'];
 
 export function AnalyticsPage() {
   const [days, setDays] = useState(30);
+  const chartTheme = useChartTheme();
 
   const { data, isLoading, isError, refetch } = useQuery({
     queryKey: ['analytics', days],
@@ -160,11 +162,7 @@ export function AnalyticsPage() {
                   ))}
                 </Pie>
                 <Tooltip
-                  contentStyle={{
-                    backgroundColor: '#1e293b',
-                    border: '1px solid rgba(99, 102, 241, 0.3)',
-                    borderRadius: '8px',
-                  }}
+                  contentStyle={chartTheme.tooltipContentStyle}
                 />
                 <Legend />
               </PieChart>
@@ -178,15 +176,11 @@ export function AnalyticsPage() {
             <h3 className="text-lg font-semibold text-text-primary mb-4">Analysis Types</h3>
             <ResponsiveContainer width="100%" height={300}>
               <BarChart data={typeData}>
-                <XAxis dataKey="name" stroke="#94a3b8" fontSize={12} />
-                <YAxis stroke="#94a3b8" fontSize={12} />
+                <XAxis dataKey="name" stroke={chartTheme.axisStroke} fontSize={12} />
+                <YAxis stroke={chartTheme.axisStroke} fontSize={12} />
                 <Tooltip
-                  contentStyle={{
-                    backgroundColor: '#1e293b',
-                    border: '1px solid rgba(99, 102, 241, 0.3)',
-                    borderRadius: '8px',
-                  }}
-                  labelStyle={{ color: '#f8fafc' }}
+                  contentStyle={chartTheme.tooltipContentStyle}
+                  labelStyle={chartTheme.tooltipLabelStyle}
                 />
                 <Bar dataKey="count" fill="#6366f1" radius={[4, 4, 0, 0]} />
               </BarChart>
@@ -222,15 +216,11 @@ export function AnalyticsPage() {
                   <h4 className="text-md font-semibold text-text-primary mb-4">Analyses per Company Over Time</h4>
                   <ResponsiveContainer width="100%" height={300}>
                     <LineChart data={pivoted}>
-                      <XAxis dataKey="month" stroke="#94a3b8" fontSize={12} />
-                      <YAxis stroke="#94a3b8" fontSize={12} />
+                      <XAxis dataKey="month" stroke={chartTheme.axisStroke} fontSize={12} />
+                      <YAxis stroke={chartTheme.axisStroke} fontSize={12} />
                       <Tooltip
-                        contentStyle={{
-                          backgroundColor: '#1e293b',
-                          border: '1px solid rgba(99, 102, 241, 0.3)',
-                          borderRadius: '8px',
-                        }}
-                        labelStyle={{ color: '#f8fafc' }}
+                        contentStyle={chartTheme.tooltipContentStyle}
+                        labelStyle={chartTheme.tooltipLabelStyle}
                       />
                       <Legend />
                       {companies.map((company, idx) => (
@@ -268,15 +258,11 @@ export function AnalyticsPage() {
                   <h4 className="text-md font-semibold text-text-primary mb-4">Analysis Types Over Time</h4>
                   <ResponsiveContainer width="100%" height={300}>
                     <BarChart data={pivoted}>
-                      <XAxis dataKey="month" stroke="#94a3b8" fontSize={12} />
-                      <YAxis stroke="#94a3b8" fontSize={12} />
+                      <XAxis dataKey="month" stroke={chartTheme.axisStroke} fontSize={12} />
+                      <YAxis stroke={chartTheme.axisStroke} fontSize={12} />
                       <Tooltip
-                        contentStyle={{
-                          backgroundColor: '#1e293b',
-                          border: '1px solid rgba(99, 102, 241, 0.3)',
-                          borderRadius: '8px',
-                        }}
-                        labelStyle={{ color: '#f8fafc' }}
+                        contentStyle={chartTheme.tooltipContentStyle}
+                        labelStyle={chartTheme.tooltipLabelStyle}
                       />
                       <Legend />
                       {types.map((type, idx) => (

frontend/src/pages/Batch.tsx

@@ -3,6 +3,7 @@ import { useMutation, useQuery } from '@tanstack/react-query';
 import { analysisApi } from '../api/client';
 import { Rocket, CheckCircle, AlertCircle, ChevronDown, ChevronUp, RefreshCw, Inbox } from 'lucide-react';
 import { BarChart, Bar, XAxis, YAxis, Tooltip, ResponsiveContainer, Cell } from 'recharts';
+import { useChartTheme } from '../context/useChartTheme';
 import type { BatchAnalysisResult } from '../types';
 
 export function Batch() {
@@ -12,6 +13,8 @@ export function Batch() {
   const [result, setResult] = useState<BatchAnalysisResult | null>(null);
   const [expandedItems, setExpandedItems] = useState<Set<string>>(new Set());
 
+  const chartTheme = useChartTheme();
+
   const modelsQuery = useQuery({
     queryKey: ['models'],
     queryFn: () => analysisApi.listModels(),
@@ -210,15 +213,11 @@ export function Batch() {
             <div className="bg-bg-card/60 border border-primary/15 rounded-2xl p-6">
               <ResponsiveContainer width="100%" height={300}>
                 <BarChart data={chartData}>
-                  <XAxis dataKey="name" stroke="#94a3b8" fontSize={12} />
-                  <YAxis stroke="#94a3b8" fontSize={12} />
+                  <XAxis dataKey="name" stroke={chartTheme.axisStroke} fontSize={12} />
+                  <YAxis stroke={chartTheme.axisStroke} fontSize={12} />
                   <Tooltip
-                    contentStyle={{
-                      backgroundColor: '#1e293b',
-                      border: '1px solid rgba(99, 102, 241, 0.3)',
-                      borderRadius: '8px',
-                    }}
-                    labelStyle={{ color: '#f8fafc' }}
+                    contentStyle={chartTheme.tooltipContentStyle}
+                    labelStyle={chartTheme.tooltipLabelStyle}
                   />
                   <Bar dataKey="patents" radius={[4, 4, 0, 0]}>
                     {chartData.map((entry, index) => (