feat(editor): replace Slate with ProseMirror in the composer

[eleboucher]

Description

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

[github-actions]

Deploying with    Cloudflare Workers

Status	Preview URL	Commit	Alias	Updated (UTC)
✅ Deployment successful!	https://pr-1724-sable.raspy-dream-bb1d.workers.dev	e89b1b8	pr-1724	Tue, 11 Aug 2026 16:47:55 GMT

[Th-Underscore]

The composer can't insert line breaks, so Enter/Shift+Enter do nothing. ProseMirrorEditable.tsx:41 skips insertNewline() when event.defaultPrevented, but ProseMirror's own keydown handler calls preventDefault() on every Enter (see captureKeyDown in prosemirror-view, which returns true for keyCode 13). The keymap in prosemirrorController.ts:123 withholding Enter doesn't help either.

Also, the tests pass because jsdom keydowns have no keyCode; a real browser reproduces it.