Content CSS Phase 7: remove the Plate media nodes from Aurora's presets

[sneridagh]

Phase 7 of #200, changed to a removal: Aurora's presets no longer load Plate's media nodes, so there's nothing to convert. Same treatment as the native img node in #201.

Stacked on #214 (Phase 6). The base is b6-table. This PR's own change is only the last commit.

Why

The plan was to move the video, audio, file, media embed and caption styles to styles/content.css. But the media nodes can't be reached in Aurora:

• No way to insert them: neither the slash menu nor the floating toolbar of the somersault-editor preset offers them. The fixed toolbar that does is only in the stock full preset.
• Pasted iframes break silently: pasting HTML with an <iframe> made a media_embed. It showed in the editor, but the renderer kit has no component for it, so the public view didn't show the embed.
• File drops have no backend: dropping or pasting non-image files went to Plate's upload placeholder, which needs an upload backend Aurora doesn't have. Image files are already taken over by PloneImageUploadPlugin, which makes Plone image blocks.

Aurora uses Plone blocks for media (image, video…).

Changes

• BlockEditorKit and BlockBaseEditorKit (used by the somersault-* and block-* presets) no longer include MediaKit / BaseMediaKit. That removes the video, audio, file, media embed, upload placeholder and caption plugins.
• config.blocks.plateBlocksConfig loses its video, audio, file and media_embed entries.
• PloneImageUploadPlugin's comment says the media placeholder flow it takes over from only exists in other presets now.
• Kept: the media components and kits. The stock full preset (EditorKit / BaseEditorKit) still uses them, so they stay on Tailwind. That's fine, since plate outside Aurora doesn't need the content contract.

Behavior changes

Action	Before	After
Paste HTML with <iframe>	media_embed node; shown in the editor, missing in the public view	The iframe is dropped; the surrounding text is kept
Paste HTML with <video> / <audio>	Dropped	Dropped (unchanged)
Drop or paste a non-image file	Upload placeholder, then a failed upload	Nothing is inserted
Drop or paste an image file	Plone image block	Plone image block (unchanged)

Nothing in Aurora could create video, audio or file nodes, so no stored content should have them. A media_embed saved from an earlier iframe paste would now render as an empty element in the editor too, as it already did publicly.

Tests

• New: paste.test.ts › "Pasting HTML with embedded media keeps only its text": pastes a heading, paragraphs, an iframe, a video and an audio, and checks that only the heading and paragraphs remain.

Validation

• CI=1 pnpm visual-test --retries=0: 24/24 pixel-identical.
• pnpm acceptance-test: 177 passed, including the new paste test.
• pnpm --filter @plone/plate test --run: 75 passed. pnpm --filter @plone/blocks test --run: 29 passed. check:ts (plate, blocks), eslint and prettier: clean.

Part of #200.