Use semantic landmarks for comments wrapper and compose form

[lrdj]

What

Two small semantic-HTML changes to improve screen-reader navigation:

• src/client.ts: the wrapper injected around the comments iframe is now
  <aside aria-label="Comments"> instead of <div class="utterances">. This
  exposes the comments widget as a named complementary landmark, so AT users can
  jump straight to it from the host page's landmark list.
• src/new-comment-component.ts: the compose-form root is now <section aria-label="Write a comment"> instead of <article>. An unsent draft isn't a
  self-contained published composition, so <article> was semantically
  incorrect — <section> with a label is the right fit. Existing comments
  rendered in CommentComponent remain <article>, which is correct.

Why

Both elements kept the .utterances / .timeline-comment classes, so there
is no visual or styling impact and no change to the public embed API. The
benefit is purely for users of assistive technology and for the document
outline.

Refs

• WAI-ARIA Authoring Practices — [complementary landmark](https://www.w3.org/W
  AI/ARIA/apg/practices/landmark-regions/#complementary)
• HTML spec — [the <article> element](https://html.spec.whatwg.org/multipage
  /sections.html#the-article-element) (self-contained composition)

Testing

Verified:

• grep confirms no selector in the project depends on the changed tag names
  (div.utterances, article.timeline-comment) — everything uses class
  selectors, which are unchanged.
• TypeScript: the only type change is script.nextElementSibling as HTMLElement (was HTMLDivElement), a widening cast that remains valid.

Not verified:

• I was unable to run the dev server locally to confirm rendered output. The
  current lmdb native dependency (pulled in via parcel@2.3.x) fails to build
  against modern Node (Node 24), with V8 API errors from an older nan.
  Working around this would have required setting up an older Node toolchain
  just for this two-line change, which felt disproportionate. Happy to do so on
  request, or to validate against CI output if available.