feat: add QR code generator tool

Add a sixth tool with live SVG preview, customizable foreground/background
colors, error correction level, margin control, and export as PNG (256–2048px)
or SVG. URL params enable shareable state. All processing runs client-side
via the qrcode package.

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

components/qrcode/QRInput.tsx

@@ -0,0 +1,34 @@
+'use client';
+
+import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card';
+import { Textarea } from '@/components/ui/textarea';
+
+interface QRInputProps {
+  value: string;
+  onChange: (value: string) => void;
+}
+
+const MAX_LENGTH = 2048;
+
+export function QRInput({ value, onChange }: QRInputProps) {
+  return (
+    <Card>
+      <CardHeader>
+        <CardTitle>Text</CardTitle>
+      </CardHeader>
+      <CardContent className="space-y-2">
+        <Textarea
+          value={value}
+          onChange={(e) => onChange(e.target.value)}
+          placeholder="Enter text or URL..."
+          maxLength={MAX_LENGTH}
+          rows={3}
+          className="resize-none font-mono text-sm"
+        />
+        <div className="text-[10px] text-muted-foreground text-right">
+          {value.length} / {MAX_LENGTH}
+        </div>
+      </CardContent>
+    </Card>
+  );
+}