Colorize: Add dark mode with system/dark/light toggle

- Add prefers-color-scheme: dark media query for automatic dark mode
- Add data-theme attribute for manual override (persisted to localStorage)
- 3-state cycle: system → dark → light → system
- Theme toggle button in nav with contextual icon (sun/moon/half)
- Dark palette: slate-900 bg, slate-800 surfaces, adjusted text/accent/success/danger

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

src/App.css

@@ -33,6 +33,70 @@
   --radius: 12px;
 }
 
+/* Dark mode */
+@media (prefers-color-scheme: dark) {
+  :root:not([data-theme="light"]) {
+    --bg: #0f172a;
+    --surface: #1e293b;
+    --border: #334155;
+    --text-primary: #f1f5f9;
+    --text-secondary: #cbd5e1;
+    --text-muted: #94a3b8;
+    --accent: #3b82f6;
+    --primary: #3b82f6;
+    --accent-light: #1e3a5f;
+    --success: #34d399;
+    --success-light: #064e3b;
+    --danger: #f87171;
+    --danger-light: #7f1d1d;
+    --brand-icon: #60a5fa;
+    --brand-text: #93c5fd;
+    --text-inverse: #0f172a;
+    --warning-bg: #451a03;
+    --warning-text: #fbbf24;
+    --warning-border: #78350f;
+    --accent-hover: #60a5fa;
+    --purple: #a78bfa;
+    --muted-light: #1e293b;
+    --dark-surface: #0f172a;
+    --dark-muted: #64748b;
+    --shadow-sm: 0 1px 2px rgba(0,0,0,0.3);
+    --shadow: 0 4px 12px rgba(0,0,0,0.4);
+    color-scheme: dark;
+  }
+}
+
+/* Manual theme override */
+:root[data-theme="dark"] {
+  --bg: #0f172a;
+  --surface: #1e293b;
+  --border: #334155;
+  --text-primary: #f1f5f9;
+  --text-secondary: #cbd5e1;
+  --text-muted: #94a3b8;
+  --accent: #3b82f6;
+  --primary: #3b82f6;
+  --accent-light: #1e3a5f;
+  --success: #34d399;
+  --success-light: #064e3b;
+  --danger: #f87171;
+  --danger-light: #7f1d1d;
+  --brand-icon: #60a5fa;
+  --brand-text: #93c5fd;
+  --text-inverse: #0f172a;
+  --warning-bg: #451a03;
+  --warning-text: #fbbf24;
+  --warning-border: #78350f;
+  --accent-hover: #60a5fa;
+  --purple: #a78bfa;
+  --muted-light: #1e293b;
+  --dark-surface: #0f172a;
+  --dark-muted: #64748b;
+  --shadow-sm: 0 1px 2px rgba(0,0,0,0.3);
+  --shadow: 0 4px 12px rgba(0,0,0,0.4);
+  color-scheme: dark;
+}
+
 * {
   margin: 0;
   padding: 0;