Put the consumption page introduction on a readable card

The World consumption heading and introduction were dark text directly over the illustrated background. Put both inside the existing white card style, with padding matching the report on desktop and mobile.

Related to #22 (closed) and #26 (closed); follows !76 (merged).

Validation: all 21 frontend tests pass and the production frontend build succeeds (existing unused-variable warnings remain). Rendered the built page with isolated synthetic report data at desktop and 390px mobile widths: the heading and introduction sit on an opaque white panel, wrap within it, and introduce no horizontal page overflow. No report or gameplay logic changes.