WEB-4512 Rework footer with AI provider links and live-site visual parity

The footer banner adds icon links that open ChatGPT, Claude, Gemini,
Grok, or Perplexity prefilled with a GoodData overview prompt, plus
layout and a11y tweaks: logo/subtitle wrap, subscribe-action sublinks
filtered out, aria-labels on icon-only social buttons.

The learn-assets Footer.css the theme loads is older than the live
site's footer styles, so local overrides bring visuals in line with
www.gooddata.ai: 48px icon buttons in a 12px-gap left-aligned row,
live text rhythm for the subtitle, and a full-width banner item with
80px vertical padding.

Author: czechian