Add dark mode toggle via next-themes

Uses next-themes with system default. Toggle button in sidebar
switches between light/dark and persists to localStorage.
CSS variables for dark mode were already defined in globals.css.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -18,6 +18,7 @@
     "lucide-react": "^1.8.0",
     "next": "16.2.4",
     "next-auth": "^5.0.0-beta.31",
+    "next-themes": "^0.4.6",
     "papaparse": "^5.5.3",
     "pg": "^8.20.0",
     "react": "19.2.4",