pdfTheme: Add dark mode theme variant with WCAG AA contrast compliance

[Kingsman-99]

Description

pdfTheme defines a single light colour scheme for PDF exports. Contributors should add a darkTheme export that uses a dark background with light text, and update the PDF generation caller to select the theme based on a preferDark parameter, so users who work in dark mode can get matching PDF receipts.

Acceptance Criteria

• A darkTheme object is exported from src/lib/pdfTheme.ts with background, text, and accent colour definitions
• The dark theme passes WCAG AA contrast ratio requirements (verified by src/lib/contrast.ts)
• PDF generation code accepts a preferDark: boolean parameter and applies the correct theme
• Existing tests continue to pass

Context

• Target file: src/lib/pdfTheme.ts
• Contrast checker: src/lib/contrast.ts