fix: stop forcing color-scheme light so native controls follow the OS

[MaximeGaudin]

Summary

• styles.css set color-scheme: light on :root with a single hardcoded light palette and no prefers-color-scheme handling, so the site forced light-themed native controls (scrollbars, form widgets) even for visitors in dark mode.
• The docs design is a deliberately crafted light-only "cool mist + ink console" palette (978 lines of light-theme CSS), so this takes the issue's first resolution: commit to light-only and drop the color-scheme: light forcing. With no color-scheme set, the page palette stays exactly as designed while native widget theming follows the visitor's OS preference.
• A short comment records the intentional omission so it is not "fixed" back later.

Fixes #141

Test plan

• pnpm run build in apps/docs: 48 pages built successfully.
• pnpm run lint (biome + astro check + check-no-todos): 0 errors, 0 warnings, clean.
• CI green on the PR