Add configurable PDF and PNG label exports

[bokicks283]

Summary

• Add export controls for choosing between individual labels and printable sheets.
• Add PDF and PNG format options.
• Preserve the existing printable A4 PDF behavior as the default.
• Generate one exact-size PDF or PNG file per label in individual mode.
• Package multiple individual files into a ZIP archive with safe, unique filenames.
• Generate transparent 300-DPI PNG output with embedded physical-size metadata.
• Leave PDF pages unpainted so no explicit white label background is added.
• Support multiple PNG sheet pages through ZIP packaging.
• Keep borders inside the physical label boundaries.
• Constrain long text to the area available beside the logo.
• Support browser-compatible uploaded logo formats in PDF and PNG exports.
• Show progress and prevent duplicate export clicks while files are generated.
• Document the new export workflow.

Verification

• pnpm lint
• pnpm build
• git diff --cached --check
• Confirmed a 40 x 12 mm individual PDF has an exact 40 x 12 mm page.
• Rendered the exact-size PDF and confirmed its border remains inside the page.
• Confirmed PNG output uses RGBA transparency.
• Confirmed PNG output includes approximately 300-DPI sizing metadata.
• Confirmed multi-file ZIP archives can be generated and reopened successfully.

Notes

PDF pages are left unpainted, although many PDF viewers display unpainted pages as white. PNG exports contain true alpha transparency.

The printable-sheet PDF remains available for users who prefer the existing print-and-cut workflow.

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NOTE: These changes were made with Codex assistance. I will personally review the code before marking this pull request ready for review.