feat: implement v0.4.0 - Reports, Charts, Comparison, Dark Mode, E2E Testing

Backend (@backend-dev):
- Add ReportService with PDF/CSV generation (reportlab, pandas)
- Implement Report API endpoints (POST, GET, DELETE, download)
- Add ReportRepository and schemas
- Configure storage with auto-cleanup (30 days)
- Rate limiting: 10 downloads/minute
- Professional PDF templates with charts support

Frontend (@frontend-dev):
- Integrate Recharts for data visualization
- Add CostBreakdown, TimeSeries, ComparisonBar charts
- Implement scenario comparison page with multi-select
- Add dark/light mode toggle with ThemeProvider
- Create Reports page with generation form and list
- Add new UI components: checkbox, dialog, tabs, label, skeleton
- Implement useComparison and useReports hooks

QA (@qa-engineer):
- Setup Playwright E2E testing framework
- Create 7 test spec files with 94 test cases
- Add visual regression testing with baselines
- Configure multi-browser testing (Chrome, Firefox, WebKit)
- Add mobile responsive tests
- Create test fixtures and helpers
- Setup GitHub Actions CI workflow

Documentation (@spec-architect):
- Create detailed kanban-v0.4.0.md with 27 tasks
- Update progress.md with v0.4.0 tracking
- Create v0.4.0 planning prompt

Features:
✅ PDF/CSV Report Generation
✅ Interactive Charts (Pie, Area, Bar)
✅ Scenario Comparison (2-4 scenarios)
✅ Dark/Light Mode Toggle
✅ E2E Test Suite (94 tests)

Dependencies added:
- Backend: reportlab, pandas, slowapi
- Frontend: recharts, date-fns, @radix-ui/react-checkbox/dialog/tabs
- Testing: @playwright/test

27 tasks completed, 100% v0.4.0 implementation

frontend/src/providers/ThemeProvider.tsx

@@ -0,0 +1,80 @@
+import { createContext, useContext, useEffect, useState } from 'react';
+import type { ReactNode } from 'react';
+
+type Theme = 'dark' | 'light' | 'system';
+
+interface ThemeContextType {
+  theme: Theme;
+  setTheme: (theme: Theme) => void;
+  resolvedTheme: 'dark' | 'light';
+}
+
+const ThemeContext = createContext<ThemeContextType | undefined>(undefined);
+
+const STORAGE_KEY = 'mockup-aws-theme';
+
+interface ThemeProviderProps {
+  children: ReactNode;
+  defaultTheme?: Theme;
+}
+
+export function ThemeProvider({ children, defaultTheme = 'system' }: ThemeProviderProps) {
+  const [theme, setThemeState] = useState<Theme>(() => {
+    if (typeof window !== 'undefined') {
+      const stored = localStorage.getItem(STORAGE_KEY) as Theme;
+      return stored || defaultTheme;
+    }
+    return defaultTheme;
+  });
+
+  const [resolvedTheme, setResolvedTheme] = useState<'dark' | 'light'>('light');
+
+  useEffect(() => {
+    const root = window.document.documentElement;
+    const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
+
+    const applyTheme = () => {
+      let resolved: 'dark' | 'light';
+      
+      if (theme === 'system') {
+        resolved = mediaQuery.matches ? 'dark' : 'light';
+      } else {
+        resolved = theme;
+      }
+
+      setResolvedTheme(resolved);
+      
+      if (resolved === 'dark') {
+        root.classList.add('dark');
+      } else {
+        root.classList.remove('dark');
+      }
+    };
+
+    applyTheme();
+
+    if (theme === 'system') {
+      mediaQuery.addEventListener('change', applyTheme);
+      return () => mediaQuery.removeEventListener('change', applyTheme);
+    }
+  }, [theme]);
+
+  const setTheme = (newTheme: Theme) => {
+    setThemeState(newTheme);
+    localStorage.setItem(STORAGE_KEY, newTheme);
+  };
+
+  return (
+    <ThemeContext.Provider value={{ theme, setTheme, resolvedTheme }}>
+      {children}
+    </ThemeContext.Provider>
+  );
+}
+
+export function useTheme() {
+  const context = useContext(ThemeContext);
+  if (context === undefined) {
+    throw new Error('useTheme must be used within a ThemeProvider');
+  }
+  return context;
+}