Add plain text option to markdown editor code blocks

[SylvainChevalier]

Adds a "Plain text" option to the code block language selector in the markdown editor.

Changes

• Added "Plain text" to the language selector dropdown
• Added aliases (text, plaintext, plain) for plain text code blocks
• Modified rendering logic to skip syntax highlighting for plain text
• Plain text code blocks don't add anything after backticks

Fixes #3917

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Generated with Claude Code

[SylvainChevalier]

@claude make plaintext the default when adding a new codeblock.

[claude]

Claude finished @SylvainChevalier's task in 1m 6s —— View job

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Done! ✓

Tasks

• Read repository guidelines (CLAUDE.md)
• Read the changed files to understand current implementation
• Find where default language is set for new code blocks
• Change default to plaintext
• Commit and push changes

Summary

Changed the default code block language from TypeScript ("ts") to Plain text ("text") in the markdown editor. When users now add a new code block, it will default to plain text instead of TypeScript.

Modified file:

• front_end/src/components/markdown_editor/initialized_editor.tsx:272

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[ncarazon]

lgtm