Move the list styles to styles/content.css (#200 Phase 4)

[sneridagh]

Phase 4 of #200: lists are styled by plain CSS instead of Tailwind utilities, with zero visual change.

Stacked on #210 (Phase 3). The base is b3-leaves-inlines. This PR's own change is only the last commit.

Changes

@plone/plate/styles/content.css now styles lists in both the public renderer and the editor. It uses the list hooks added in Phase 1a: data-list-style-type on the block, and the block-p__list / block-p__item parts.

Element	Hook	Before (Tailwind)
ul / ol	.block-p__list	relative m-0 p-0
To-do item	[data-list-style-type='todo'] .block-p__item	list-none
Checked to-do item	.block-p__item[data-checked] (new attribute)	text-muted-foreground line-through
Read-only checkbox (rendered content)	.block-p__checkbox, [data-state='checked'] (new part)	peer pointer-events-none absolute top-1 -left-6 size-4 shrink-0 rounded-sm border border-primary bg-background …, data-[state=checked]:bg-primary data-[state=checked]:text-primary-foreground
Its check icon	.block-p__checkmark, .block-p__checkmark svg (new part)	flex items-center justify-center text-current, size-4

• Checkbox: the read-only checkbox is a <button>, so its rule also neutralizes the browser's button styling (appearance, padding, margin, font, color). That comes from Tailwind's reset today, and a public theme might not have one.
• Dropped classes with no visual effect:
  • peer and ring-offset-background;
  • the focus-visible: ring variants: the button has pointer-events: none, and the public view never shows keyboard focus on it in practice.
• The checkbox stays keyboard-focusable, as before. Changing that is out of scope here.
• The editor's to-do checkbox is an interactive editor control (a Radix Checkbox), so it stays Tailwind. Both renderers set data-checked on checked items.

Tests

• New: plate/acceptance/tests/lists-content-css.test.ts compares the computed values of every property these rules set (list margins and padding, to-do item style, both checkbox states, the check icon size), in the public view. It runs them under Agave's preflight, with no reset, and under @plone/theming's simple reset.
• Shared helpers: the reset tests now share measureOwnedStyles() and SIMPLE_RESET from tooling/playwright/resets.ts. The Phase 2 and 3 tests no longer carry their own copies.
• Ratchet: PENDING loses the whole p entry, which was all list markup.

Validation

• CI=1 pnpm visual-test --retries=0: 24/24 pixel-identical. The native-blocks screenshots include bulleted, nested, numbered and to-do lists in both states, in the public view and the editor.
• pnpm acceptance-test: 174 passed.
• pnpm --filter @plone/plate test --run: 75 passed. check:ts, eslint and stylelint: clean.

Part of #200.