Add media embed resizing feature

[jacek-cke]

🚀 Summary

• Adds the MediaEmbedResize plugin: drag handles to resize embedded media, mirroring the image resize UX. Bundled with a resizeMediaEmbed command, supporting CSS, and plugin exports.

• Modernizes provider iframe HTML (YouTube, Vimeo, Dailymotion, Spotify) to useaspect-ratioCSS instead of a <div style="padding-bottom:…"> wrapper. Existing content still upcasts correctly via the URL. This is a prerequisite for resize to work cleanly.

• Adds an isResizable provider option (exposed via MediaRegistry.isMediaResizable()) so providers can opt out of handles — used by Spotify, whose player doesn't adapt well to free resizing.

• Splits media-embed.md into a landing page plus dedicated pages for configuration, external preview, styles, and resize. Umberto redirects and @glink references updated.

---

📌 Related issues

• Closes Media embed resizing #6593

---

💡 Additional information

The iframe HTML emitted by the built-in providers (YouTube, Vimeo, Dailymotion, Spotify) changed from a <div style="padding-bottom:…"><iframe>…</iframe></div> wrapper — which used the classic padding-bottom hack to enforce an intrinsic aspect ratio — to a bare <iframe width="…" height="…" style="width:100%; height:auto; aspect-ratio:X/Y">. This was needed because the old wrapper fought the resize mechanism (the figure's width couldn't propagate cleanly through a padding-based ratio box), and modern aspect-ratio CSS is baseline-supported (Chrome 88+, Firefox 89+, Safari 15+).

Consequences to be aware of:

• editor.getData() output for new or re-inserted embeds changes structurally — any downstream HTML post-processing, sanitizer rules, CSS selectors, or content-diffing that targeted the outer <div> wrapper will need updating.
• Existing stored content is unaffected at load time because upcast reads only the data-oembed-url and regenerates the preview from the current provider config. Once that content is re-saved, however, it persists in the new format.
• In very old browsers without aspect-ratio support, the iframe falls back to its intrinsic width/height attributes rather than preserving the ratio — acceptable for the platforms we target, but worth noting for anyone on legacy stacks.
• Custom provider definitions supplied via config.mediaEmbed.providers are not affected — they keep whatever HTML their html() function returns.

---

🧾 Checklists

Use the following checklists to ensure important areas were not overlooked.
This does not apply to feature-branch merges.
If an item is not relevant to this type of change, simply leave it unchecked.

Author checklist

• Is the changelog entry intentionally omitted?
• Is the change backward-compatible?
• Have you considered the impact on different editor setups and core interactions? (e.g., classic/inline/multi-root/many editors, typing, selection, paste, tables, lists, images, collaboration, pagination)
• Has the change been manually verified in the relevant setups?
• Does this change affect any of the above?
• Is performance impacted?
• Is accessibility affected?
• Have tests been added that fail without this change (against regression)?
• Have the API documentation, guides, feature digest, and related feature sections been updated where needed?
• Have metadata files (ckeditor5-metadata.json) been updated if needed?
• Are there any changes the team should be informed about (e.g. architectural, difficult to revert in future versions or having impact on other features)?
• Were these changes documented (in Logbook)?

Reviewer checklist

• PR description explains the changes and the chosen approach (especially when performance, API, or UX is affected).
• The changelog entry is clear, user‑ or integrator-facing, and it describes any breaking changes.
• All new external dependencies have been approved and mentioned in LICENSE.md (if any).
• All human-readable, translateable strings in this PR been introduced using t() (if any).
• I manually verified the change (e.g., in manual tests or documentation).
• The target branch is correct.

[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit ed5b325. Configure here.}