fix: restore the quotation environment's inset

[pyramation]

Summary

quotation lost its indent and 10pt size once paragraphize() started wrapping text runs in <p class="para">: the environment opened with a <p>, and isBlock matches a line starting with <p, so the opening tag was flushed as its own block and the quote body became a sibling paragraph. Rendered DOM before:

<p class="quotation"></p>          <!-- browser closes it immediately -->
<p class="para">Quoted text.</p>   <!-- p.quotation styles never apply -->

Fix is to make it a real wrapper — bq/eq emit <blockquote class="quotation">…</blockquote> — so the paragraphized lines nest inside it, with the CSS rule moved to the wrapper (same 15px inset / 0 0 20px margin / 10pt as before, inherited by the inner p.para).

That needs isBlock to match closing tags too (<\/?(h[1-6]|ul|…)), otherwise a bare </blockquote> line gets swallowed into the following paragraph run. Side effect, and an improvement: theorem-style environments no longer emit their </div> inside the paragraph.

-<p class="para">Theorem text.
-</div></p>
+<p class="para">Theorem text.</p>
+</div>

Site-visible on mathapedia.com's quoted passages (e.g. /books/31/sections/156/407) after a publish + dep bump.

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

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