contrast: Add meetsWcagAA function for WCAG 2.1 AA contrast ratio checks

[Kingsman-99]

Description

contrast.ts contains colour utilities but lacks a standardised WCAG contrast-ratio checker. Contributors should add a meetsWcagAA(foreground: string, background: string, largeText?: boolean): boolean function that calculates the relative luminance of each colour and returns whether the pair meets the WCAG 2.1 AA ratio (4.5:1 for normal text, 3:1 for large text).

Acceptance Criteria

• meetsWcagAA("#000000", "#ffffff") returns true (21:1 ratio)
• meetsWcagAA("#777777", "#ffffff") returns false for normal text (fails 4.5:1)
• meetsWcagAA accepts hex, rgb(), or rgba() colour strings
• Existing tests continue to pass

Context

• Target file: src/lib/contrast.ts
• Used by: src/lib/pdfTheme.ts

[damiedee96]

Hello Maintainer, I'd love to work on this issue.