[feature]: Add a language selector to code blocks in the editor

Is there an existing issue for this?

• I have searched the existing issues

Summary

In the Community Edition editor, a code block's language can only be set at creation time via the ```lang input rule. There is no UI to view or change it afterwards:

• Code blocks inserted via the slash command, the bubble menu, paste, or markdown import are created with language: null, so they get no syntax highlighting.
• The only way to fix a wrong or missing language is to delete the block and retype the fence from scratch.
• Read-only surfaces (pages, comments) have no way to tell which language a block was intended to use.

(#9680 references a language dropdown with a search input, but that exists only in the cloud/commercial build — CE master has no language selector at all, only the copy button in code-block-node-view.tsx.)

Proposed behavior

Add a compact language dropdown to the code block node view (top-left, editable mode only):

• Lists every grammar registered in the extension's lowlight instance, plus a "Plain text" (null) option.
• Selecting a language writes through to the node's language attribute, so lowlight highlighting updates immediately.
• An explicitly set but unregistered language stays selectable instead of blanking the picker.

I have a working implementation and would like to raise a PR if the team is interested.

Why should this be worked on?

Syntax highlighting is one of the main reasons to use code blocks, and today a large share of blocks (anything not created by typing the fence manually) silently end up unhighlighted with no recourse. A selector makes the existing lowlight integration actually reachable and matches the UX of the cloud version.

Start with code-block-node-view.tsx and trace the Community Edition code block node view, its lowlight instance, and the node's language attribute. Add an editable-mode selector covering registered grammars, Plain text, and explicitly selected unregistered languages; done means selection updates highlighting while read-only surfaces remain unchanged.