More markdown improvements

[7w1]

Description

Add support for misskey-flavored markdown color definitions. E.g. $[fg.color=f00 bg.color=00ff00 red on green].
Fix single new lines after block quotes being block-quoted.
Hardened html parsing in standard input box, should no longer randomly delete text in arrow brackets (unless valid, properly closed, legal html).
Fix matrix.to links getting arrow brackets inserted when editing messages.
Fix mentions breaking after editing messages with mentions. Fixes #858

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).

Tests were fully AI generated. AI helped with some of the regex which I then verified and tested.

[github-actions]

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Status	Preview URL	Commit	Alias	Updated (UTC)
✅ Deployment successful!	https://pr-860-sable.raspy-dream-bb1d.workers.dev	3ac14ab	pr-860	Mon, 18 May 2026 03:43:43 GMT