feat: Add configurable QR code sizing options (#3)

Addresses https://github.com/Donkie/Spoolman/issues/671

Changes:
- Remove fixed 50% max-width constraint on QR code container
- Add qrCodePadding setting (0-10mm, default 2mm) for white space
- Add qrCodeMaxWidth setting (0-100%, 0 = auto/fit-content)
- Add UI controls (slider + input) for both new settings
- Settings are disabled when QR code is turned off

The previous behavior forced QR codes to 50% width when label text
was shown, wasting label space on small labels. New default uses
fit-content for optimal sizing while allowing manual control.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

client/src/pages/printing/printing.tsx

@@ -22,6 +22,8 @@ export interface QRCodePrintSettings {
   showContent?: boolean;
   showQRCodeMode?: "no" | "simple" | "withIcon";
   textSize?: number;
+  qrCodePadding?: number; // in mm, default 2
+  qrCodeMaxWidth?: number; // percentage 0-100, 0 = no constraint (auto)
   printSettings: PrintSettings;
 }