Redesign the footer and make it available on `gr.Server` apps

Redesign the Gradio footer for Gradio 7, assuming many apps will be built with gr.Server and a custom frontend.

Goals

• A refreshed footer design for Blocks apps
• A way for the footer (or an equivalent) to also appear on gr.Server apps, e.g. an optional injected script/web component
• Room for new entry points such as API/agent usage (see #13916)