fix(client): WCAG AA colour contrast for ThemeToggle design tokens

[DevRushd]

Summary

Adds a proper design token system to the ThemeToggle component and ensures all colour pairs pass WCAG AA contrast ratios (4.5:1 normal text, 3:1 UI components). A CI check prevents future regressions.

Changes

client/src/index.css

• Defined colour design tokens via Tailwind v4 @theme with .dark overrides
• Consolidated duplicate :root blocks into one
• Body colour and background now reference --color-text-body / --color-bg CSS vars

client/src/components/ThemeToggle.tsx

• Replaced raw Tailwind colour classes with semantic token classes (bg-surface, border-border, text-icon, etc.)
• Fixed border contrast: light border was 1.35:1 (needs 3:1), now 4.39:1; dark border was 1.40:1, now 3.48:1
• Removed duplicate dark:bg-white/10 that was overriding dark:bg-white/5
• Removed redundant dark:text-white (was overridden by dark:text-white/80 anyway)

client/scripts/check-contrast.js (new)

Zero-dependency Node.js script that:

• Parses CSS variables from @theme, :root, and .dark blocks
• Computes effective colours with proper alpha blending
• Validates 8 foreground–background pairs against WCAG AA thresholds
• Exits with code 1 on any failure

client/design.md

Added a Colour Tokens section documenting all 7 tokens for both light and dark modes, with effective values and contrast ratios.

.github/workflows/ci.yml

Added a Check colour contrast (WCAG AA) step to the client CI job that runs node scripts/check-contrast.js. The pipeline fails if any token pair drops below threshold.

Contrast verification

Pair	Light	Dark	Threshold
Body text on background	17.40:1	12.10:1	4.5:1
Icon on button surface	13.34:1	9.12:1	4.5:1
Icon hover on hover surface	16.96:1	12.04:1	4.5:1
Border on button surface	4.39:1	3.48:1	3.0:1

Closes crackedstudio#827