fix: give raster figures the same \parskip as paragraphs and pspictures

[pyramation]

Summary

\img and \youtube were the only block-level things the renderer emits with no vertical spacing of their own — the wrapper carried an inline style and nothing else:

-<div style="width: 100%;text-align: center;"><img src="...">
+<div class="latex2js-figure"><img src="...">

.latex2js-figure {
  margin: var(--latex2js-parskip) auto;
  text-align: center;
}

That worked on the old mathapedia only by accident: images were inline there, so the line box's leading (~21px at line-height: 20px) happened to look like a paragraph gap. Any consumer running a modern CSS reset loses it — Tailwind's preflight sets img { display: block } — and a figure ends up flush against the following paragraph. Measured on mathapedia.com's rewrite, the gap below the figure on /books/31/sections/153/338 was 0px where production shows 21px; with this change it's 22.4px, i.e. the same 1.6em \parskip that .math > p.para and .pspicture already use.

So the spacing lands where the rest of the document rhythm already lives — one variable in latex2js.css — rather than in a consumer stylesheet. width: 100% from the inline style is dropped because a block div is already full width. No img rule is added: whether images are block or inline is the consuming site's reset to decide, and this fix holds either way.

bundle/latex2js.css and bundle/latex2html5.bundle.js are the tracked build outputs, regenerated.

Link to Devin session: https://app.devin.ai/sessions/e51e8ecb94d241bd8d89e65e642c3e43
Requested by: @pyramation

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