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/public/locales/en/common.json

@@ -123,7 +123,12 @@
                 "no": "No",
                 "simple": "Simple",
                 "withIcon": "With Icon"
-            }
+            },
+            "qrCodePadding": "QR Code Padding",
+            "qrCodePaddingTooltip": "White space around the QR code for better scanning reliability",
+            "qrCodeMaxWidth": "QR Code Max Width",
+            "qrCodeMaxWidthTooltip": "Maximum width of QR code as percentage of label. Set to 0 for automatic sizing.",
+            "auto": "Auto"
         },
         "spoolSelect": {
             "title": "Select Spools",