feat: add project layout, typography, and social previews

[balajmarius]

Adds project context, keyboard instructions, and a GitHub source link beside the game. The intro stays sticky on desktop and stacks above the console below 1280px; a bottom fade appears when the case extends below the viewport.

• Extract the page layout and reusable typography, with copy in copy.json through react-i18next and class composition through tailwind-merge.
• Add General Sans and Gambetta fonts, updated descriptions, and a 1200 × 630 image for Open Graph, Twitter, and structured data.
• Register Pixi elements once in the scene and remove redundant keyboard and visibility checks.

Validation: npm run check and npm run build pass. Browser checks covered starting, moving, firing, responsive columns, and fade visibility while scrolling and resizing.

The console retains its fixed width on narrow screens. Vite still reports the existing large-chunk warning.