Add h1 and hr to the Plate block anatomy (#200 Phase 1b)

[sneridagh]

Phase 1b of #200: the last Plate-native blocks without block anatomy classnames, h1 and hr, get them.

Stacked on #206 (Phase 1a). The base is b1a-content-contract-foundation. This PR's own change is the commit "Add h1 and hr to the Plate block anatomy"; the merge commits bring the stack up to date.

The stack now includes main with #207, which fixes the server-side translations that made React discard the server-rendered page on every load. With it, A3's widthRatio() retry in image-block-style-fields.test.ts is gone again (see #205).

Changes

config.blocks.plateBlocksConfig (packages/blocks/index.ts) gets two entries:

• h1: category text, with the same narrow block width as the other headings. The title block usually takes the place of an h1.
• hr: category separator.

Both now render block block-<type> category-<category> and the data-block-* attributes, like every other native block.

Visual result: one accepted diff, public view only

In the public view, the separator now gets the category spacing that @plone/layout's content-area.css already defines for .category-separator: no bottom spacing on the separator, nor on the block before it. "After the separator" moves up 12px in the native blocks view, desktop and mobile. Those are the only 2 changed screenshots; the other 20 are pixel-identical. The editor is unchanged, because content-area.css isn't loaded there yet.

Baselines: after merging, update with grep set to Native blocks in the public view.

Reordered plan

Moving the shared spacing rules from content-area.css into the shared content styles was planned for Phase 1. It's now a later phase, after the node conversions, and #200 is updated accordingly.

I tried it here first. The editor changes were the intended unification, but the public view changed too, from the blockquote down. Nodes like blockquote (my-1), table of contents (mb-1 p-0) and columns (mb-2) still put Tailwind utilities on their inner containers. Today content-area.css beats them from the custom layer. In plone-content it would sit below utilities and lose. Once those nodes are converted, the move only brings the intended editor changes. The work is kept on a local branch for that phase.

Validation

• CI=1 pnpm visual-test --retries=0: 20 identical, and 2 accepted diffs (above).
• pnpm acceptance-test: 170 passed, including the content class-contract test and Fix the server not loading translations, which broke hydration #207's hydration test. One unrelated flake in paste.test.ts › "Pasting from Word…" on the first run; it then passed 20/20 with --repeat-each=5.
• With Fix the server not loading translations, which broke hydration #207, /layers.css is still the first stylesheet in the served HTML on every route (public, login, editor, contents), and the order after hydration is the same.
• The visual results above are unchanged with Fix the server not loading translations, which broke hydration #207 merged in.
• pnpm --filter @plone/blocks test --run: 29 passed. check:ts: clean.

Part of #200.