fix(html-embed): correct stale iframe height and damp the resize loop

Address the non-test code-review findings on the htmlEmbed sandbox change
(test-coverage gaps are tracked in issue #99):

- html-embed-view: track the iframe's reported content height even while a
  fixed height is set, so clearing the height (fixed -> auto) without editing
  the source no longer leaves the frame pinned to the stale value. Derive the
  fixed-height predicate once; seed autoHeight to the default.
- html-embed-view: drop width/border from the iframe inline style (the
  .htmlEmbedFrame CSS class already provides them).
- html-embed-sandbox: coalesce height reports via requestAnimationFrame and
  skip <=1px deltas to damp the self-measure feedback loop; fix the misleading
  bootstrap comment.
- tracker-settings: add an aria-label to the snippet Textarea (a11y).
- CHANGELOG: note the removal of server-side role-based HTML-embed stripping.

apps/client/src/features/workspace/components/settings/components/tracker-settings.tsx

@@ -78,6 +78,7 @@ export default function TrackerSettings() {
           autosize
           minRows={6}
           maxRows={20}
+          aria-label={t("Analytics / tracker")}
           value={value}
           onChange={(e) => setValue(e.currentTarget.value)}
           placeholder={t("<script>...</script>")}