Sections: allow saving a block as a section from the editor

[kwmr]

Summary

Ghost lets you select a block in the editor and save it as a snippet, naming it
inline. In EmDash, sections can only be authored from the Sections admin page,
so there is no path from "I just built this block" to "reuse it later".

Current behaviour

• The per-block menu (drag handle) offers Turn into, Duplicate, Delete,
  Insert block below. There is no "save as section".
• Sections are created only via Sections → new, by authoring the content again
  in a separate editor.
• Together with Sections editor doesn't receive pluginBlocks, so plugin blocks can't be used in sections #2326 (the section editor doesn't receive pluginBlocks), a
  section cannot contain an embed at all today.

Proposal

Add Save as section to the block menu (and/or for a multi-block selection),
prompting for a name and creating a Section from the selected Portable Text.

Why it matters

Reuse is most valuable for the blocks that took effort to build — embeds, cards,
callouts — and those are exactly the ones you have just finished adjusting in
the editor. Re-authoring them on a separate page is enough friction that the
feature tends to go unused.

Note

This cannot be implemented as a plugin: the manifest exposes
portableTextBlocks, fieldWidgets, adminPages and dashboardWidgets, but
no extension point for editor or block-level actions.

[MA2153]

Converted this to a discussion as it is a feature request.