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/index.css

@@ -1,59 +1,90 @@
-@tailwind base;
-@tailwind components;
-@tailwind utilities;
+@import "tailwindcss";
 
-@layer base {
-  :root {
-    --background: 0 0% 100%;
-    --foreground: 222.2 84% 4.9%;
-    --card: 0 0% 100%;
-    --card-foreground: 222.2 84% 4.9%;
-    --popover: 0 0% 100%;
-    --popover-foreground: 222.2 84% 4.9%;
-    --primary: 222.2 47.4% 11.2%;
-    --primary-foreground: 210 40% 98%;
-    --secondary: 210 40% 96.1%;
-    --secondary-foreground: 222.2 47.4% 11.2%;
-    --muted: 210 40% 96.1%;
-    --muted-foreground: 215.4 16.3% 46.9%;
-    --accent: 210 40% 96.1%;
-    --accent-foreground: 222.2 47.4% 11.2%;
-    --destructive: 0 84.2% 60.2%;
-    --destructive-foreground: 210 40% 98%;
-    --border: 214.3 31.8% 91.4%;
-    --input: 214.3 31.8% 91.4%;
-    --ring: 222.2 84% 4.9%;
-    --radius: 0.5rem;
-  }
-
-  .dark {
-    --background: 222.2 84% 4.9%;
-    --foreground: 210 40% 98%;
-    --card: 222.2 84% 4.9%;
-    --card-foreground: 210 40% 98%;
-    --popover: 222.2 84% 4.9%;
-    --popover-foreground: 210 40% 98%;
-    --primary: 210 40% 98%;
-    --primary-foreground: 222.2 47.4% 11.2%;
-    --secondary: 217.2 32.6% 17.5%;
-    --secondary-foreground: 210 40% 98%;
-    --muted: 217.2 32.6% 17.5%;
-    --muted-foreground: 215 20.2% 65.1%;
-    --accent: 217.2 32.6% 17.5%;
-    --accent-foreground: 210 40% 98%;
-    --destructive: 0 62.8% 30.6%;
-    --destructive-foreground: 210 40% 98%;
-    --border: 217.2 32.6% 17.5%;
-    --input: 217.2 32.6% 17.5%;
-    --ring: 212.7 26.8% 83.9%;
-  }
+@theme {
+  --color-border: hsl(var(--border));
+  --color-input: hsl(var(--input));
+  --color-ring: hsl(var(--ring));
+  --color-background: hsl(var(--background));
+  --color-foreground: hsl(var(--foreground));
+  --color-primary: hsl(var(--primary));
+  --color-primary-foreground: hsl(var(--primary-foreground));
+  --color-secondary: hsl(var(--secondary));
+  --color-secondary-foreground: hsl(var(--secondary-foreground));
+  --color-destructive: hsl(var(--destructive));
+  --color-destructive-foreground: hsl(var(--destructive-foreground));
+  --color-muted: hsl(var(--muted));
+  --color-muted-foreground: hsl(var(--muted-foreground));
+  --color-accent: hsl(var(--accent));
+  --color-accent-foreground: hsl(var(--accent-foreground));
+  --color-popover: hsl(var(--popover));
+  --color-popover-foreground: hsl(var(--popover-foreground));
+  --color-card: hsl(var(--card));
+  --color-card-foreground: hsl(var(--card-foreground));
+  --radius-sm: calc(var(--radius) - 4px);
+  --radius-md: calc(var(--radius) - 2px);
+  --radius-lg: var(--radius);
 }
 
-@layer base {
-  * {
-    @apply border-border;
-  }
-  body {
-    @apply bg-background text-foreground;
-  }
+:root {
+  --background: 0 0% 100%;
+  --foreground: 222.2 84% 4.9%;
+  --card: 0 0% 100%;
+  --card-foreground: 222.2 84% 4.9%;
+  --popover: 0 0% 100%;
+  --popover-foreground: 222.2 84% 4.9%;
+  --primary: 221.2 83.2% 53.3%;
+  --primary-foreground: 210 40% 98%;
+  --secondary: 210 40% 96.1%;
+  --secondary-foreground: 222.2 47.4% 11.2%;
+  --muted: 210 40% 96.1%;
+  --muted-foreground: 215.4 16.3% 46.9%;
+  --accent: 210 40% 96.1%;
+  --accent-foreground: 222.2 47.4% 11.2%;
+  --destructive: 0 84.2% 60.2%;
+  --destructive-foreground: 210 40% 98%;
+  --border: 214.3 31.8% 91.4%;
+  --input: 214.3 31.8% 91.4%;
+  --ring: 221.2 83.2% 53.3%;
+  --radius: 0.5rem;
+}
+
+.dark {
+  --background: 222.2 84% 4.9%;
+  --foreground: 210 40% 98%;
+  --card: 222.2 84% 4.9%;
+  --card-foreground: 210 40% 98%;
+  --popover: 222.2 84% 4.9%;
+  --popover-foreground: 210 40% 98%;
+  --primary: 217.2 91.2% 59.8%;
+  --primary-foreground: 222.2 47.4% 11.2%;
+  --secondary: 217.2 32.6% 17.5%;
+  --secondary-foreground: 210 40% 98%;
+  --muted: 217.2 32.6% 17.5%;
+  --muted-foreground: 215 20.2% 65.1%;
+  --accent: 217.2 32.6% 17.5%;
+  --accent-foreground: 210 40% 98%;
+  --destructive: 0 62.8% 30.6%;
+  --destructive-foreground: 210 40% 98%;
+  --border: 217.2 32.6% 17.5%;
+  --input: 217.2 32.6% 17.5%;
+  --ring: 224.3 76.3% 48%;
+}
+
+* {
+  border-color: hsl(var(--border));
+}
+
+body {
+  background-color: hsl(var(--background));
+  color: hsl(var(--foreground));
+}
+
+/* Smooth transitions for theme switching */
+html {
+  transition: background-color 0.3s ease, color 0.3s ease;
+}
+
+/* Chart tooltip styles for dark mode */
+.dark .recharts-tooltip-wrapper {
+  filter: drop-shadow(0 4px 6px rgba(0, 0, 0, 0.3));
 }