feat: add frontend source code

Add search-zen-50 React SPA source code to frontend/ directory.
Build artifacts (dist, node_modules, lock files) are gitignored.

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

frontend/src/pages/Preferences.tsx

@@ -0,0 +1,88 @@
+import { useNavigate } from "react-router-dom";
+import { ArrowLeft, Sun, Moon } from "lucide-react";
+import { Card, CardContent, CardDescription, CardHeader, CardTitle } from "@/components/ui/card";
+import { Switch } from "@/components/ui/switch";
+import { Label } from "@/components/ui/label";
+import { Button } from "@/components/ui/button";
+import { usePreferences } from "@/contexts/PreferencesContext";
+
+const Preferences = () => {
+  const navigate = useNavigate();
+  const { theme, engines, setTheme, toggleEngine } = usePreferences();
+
+  return (
+    <div className="min-h-screen bg-background">
+      {/* Header */}
+      <header className="sticky top-0 z-50 bg-background/95 backdrop-blur-sm border-b border-border">
+        <div className="max-w-2xl mx-auto px-4 py-3 flex items-center gap-3">
+          <Button variant="ghost" size="icon" onClick={() => navigate("/")} className="text-muted-foreground hover:text-foreground">
+            <ArrowLeft size={18} />
+          </Button>
+          <h1 className="font-mono font-bold text-lg text-foreground tracking-tight">Preferences</h1>
+        </div>
+      </header>
+
+      <main className="max-w-2xl mx-auto px-4 py-8 space-y-6">
+        {/* Appearance */}
+        <Card className="bg-card border-border">
+          <CardHeader>
+            <CardTitle className="text-foreground">Appearance</CardTitle>
+            <CardDescription className="text-muted-foreground">Choose your preferred theme.</CardDescription>
+          </CardHeader>
+          <CardContent>
+            <div className="flex gap-3">
+              <button
+                onClick={() => setTheme("light")}
+                className={`flex-1 flex items-center justify-center gap-2 px-4 py-3 rounded-lg border transition-colors ${
+                  theme === "light"
+                    ? "border-primary bg-primary/10 text-foreground"
+                    : "border-border text-muted-foreground hover:border-muted-foreground/40"
+                }`}
+              >
+                <Sun size={18} />
+                <span className="text-sm font-medium">Light</span>
+              </button>
+              <button
+                onClick={() => setTheme("dark")}
+                className={`flex-1 flex items-center justify-center gap-2 px-4 py-3 rounded-lg border transition-colors ${
+                  theme === "dark"
+                    ? "border-primary bg-primary/10 text-foreground"
+                    : "border-border text-muted-foreground hover:border-muted-foreground/40"
+                }`}
+              >
+                <Moon size={18} />
+                <span className="text-sm font-medium">Dark</span>
+              </button>
+            </div>
+          </CardContent>
+        </Card>
+
+        {/* Search Engines */}
+        <Card className="bg-card border-border">
+          <CardHeader>
+            <CardTitle className="text-foreground">Search Engines</CardTitle>
+            <CardDescription className="text-muted-foreground">Enable or disable engines used for queries.</CardDescription>
+          </CardHeader>
+          <CardContent>
+            <div className="space-y-4">
+              {engines.map((engine) => (
+                <div key={engine.id} className="flex items-center justify-between">
+                  <Label htmlFor={engine.id} className="text-foreground font-medium cursor-pointer">
+                    {engine.name}
+                  </Label>
+                  <Switch
+                    id={engine.id}
+                    checked={engine.enabled}
+                    onCheckedChange={() => toggleEngine(engine.id)}
+                  />
+                </div>
+              ))}
+            </div>
+          </CardContent>
+        </Card>
+      </main>
+    </div>
+  );
+};
+
+export default Preferences;