feat: Add QR code label dimensions and printing settings

[Ajimaru]

This pull request adds customizable label size settings for QR code printing and improves the QR label preview and printing workflow in the SpoolManager plugin. Users can now set the physical dimensions of the printed label, and the QR label preview includes a print button and better layout for printing.

New QR label size customization:

• Added new settings qrCodeLabelWidthMM and qrCodeLabelHeightMM for specifying label width and height in millimeters, with defaults set to 89×36 mm (Dymo 99012 address label). These are exposed in the settings UI and used when printing QR labels. [1] [2] [3]

Improved QR label preview and printing:

• The QR label HTML view now includes a "Print Label" button and styles that format the page correctly for the chosen label size when printing. The layout adapts for print media, hiding the print button and sizing the content for the label.
• Added a "Preview Label" button to the spool edit dialog in simple mode, allowing users to preview and print the QR label for an existing spool.

Related Issues & PR's:

• Feature Request: Printing QR Code Spool Label #47
  
• Print QR Code Functionality#54
  
• adding print preview button#59