chore(i18n): sync main into translation/jp

[thetaPC]

Issue URL: N/A

What is the current behavior?

translation/jp is three commits behind main, and the Japanese site has 143 broken anchors and 1 broken link. Nearly all of it is one cause: main pinned an explicit id on every English heading in #4742, but the translated copies keep their own heading text, so the generated slug no longer matches the link. 103 of the anchors are the single heading ### 仮想プロパティ.

What is the new behavior?

Merges main into translation/jp, taking ja to 0 broken anchors and 0 broken links.

• 840 heading conflicts resolved the same way: Japanese text, English id, as in ## UI コンポーネント {/* #ui-components */}
• 32 heading levels realigned to main, 26 of them ## CSSカスタムプロパティ to ###
• 13 pages deleted, following main removing them in chore(cleanup): remove unreferenced files and redirect ja developer-resource #4741
• docs/api/item.mdx linked to ](/#clickable-items), where the leading slash pointed at the site root instead of the page

Does this introduce a breaking change?

• Yes
• No

Other information

Merge this with a merge commit, not a squash. A squash destroys the merge base and every later sync re-conflicts on the whole tree, which is what #4729 had to repair.

The heading levels were the only judgement call. Main never changed them; jp had translated those headings at the wrong depth. Realigning fixes the drift and stops the same 32 conflicts recurring on every future sync, at the cost of changing TOC nesting on those pages. The anchors are fixed either way, since the ids are explicit.

Verified by building main with this branch's content as its ja locale and onBrokenAnchors: 'throw' enabled. This branch's own preview cannot show any of it: jp builds as a single locale at /docs/, so the doubled-baseUrl and slug-mismatch problems never appear there.

57 headings remain unpinned, all in five jp-only files (react.mdx, layout/grid.mdx, deployment/desktop-app.mdx, developer-resources/books.mdx, studio/setup/installation.mdx). None exists on main, and the docs plugin globs the English docs/, so they never render on the production ja site.

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