feat(toolrate): light/dark/system theme toggle with FOUC guard

- use-theme hook: localStorage (toolrate-theme, default system), matchMedia
  change listener, shared singleton listener, sets .dark + color-scheme
- ThemeToggle: lightbulb quick toggle (light/dark) + dropdown (light/dark/system)
- Sidebar footer + mobile header + standalone login page
- inline script in index.html to apply theme pre-render (no FOUC)
- recharts axis ticks use hsl(var(--foreground)) for dark-mode readability

artifacts/toolrate/src/pages/login.tsx

@@ -6,6 +6,7 @@ import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
 import { Card, CardContent, CardHeader, CardTitle, CardDescription } from "@/components/ui/card";
+import { ThemeToggle } from "@/components/theme-toggle";
 import { Wrench, AlertCircle } from "lucide-react";
 
 export default function Login() {
@@ -40,6 +41,9 @@ export default function Login() {
 
   return (
     <div className="min-h-screen bg-background flex items-center justify-center p-4">
+      <div className="absolute top-4 right-4">
+        <ThemeToggle />
+      </div>
       <div className="w-full max-w-sm space-y-6">
         <div className="flex flex-col items-center gap-2 text-center">
           <div className="flex items-center gap-2 text-primary font-bold text-2xl">