fix(docs): constrain content images and shrink demo GIF

[aheritier]

Problem

On the TUI feature page, the demo GIF renders oversized and overflows the content column (pushing past the table of contents).

Two root causes:

• The page embeds docs/demo.gif (intrinsic 2000×1600px) as a bare Markdown image.
• docs/css/style.css had no general content-image constraint — only scoped .flow-diagram img and .demo-container img/video rules — so the GIF rendered at native 2000px.

Changes

• docs/css/style.css — add a catch-all responsive rule so every in-article image scales to its container:

  .content img {
    max-width: 100%;
    height: auto;
  }

• docs/demo.gif — re-encoded to reduce page weight (kept as a GIF):
  • 2000×1600 → 1280×1024 (2× the ~640px column, retina-crisp)
  • 25fps → 12.5fps
  • 1.0 MB → 555 KB (~46% smaller); full 14.7s animation preserved

Notes

The frozen .flow-diagram / .demo-container rules are untouched, and no Markdown pages were edited.