fix(desktop): editor bug fixes — codeblock escaping, thread edit leak, channel links, and edit cancel

[wesbillman]

Summary

• Codeblock escaping: prosemirror-markdown's esc() backslash-escapes markdown special characters in text nodes. Added post-processing in getMarkdownFromEditor() to unescape them so code fences, inline code, bold/italic survive the editor roundtrip.
• Thread edit leaks to main composer: A single editTarget was passed to both the main channel composer and thread panel composer. Added isEditInThread check to scope editTarget to the correct composer based on whether the message lives in the thread or main timeline.
• Channel link / @mention not rendering as sole content: createRemarkPrefixPlugin's walkChildren only spliced replacements when parts.length > 1, skipping single-match text nodes. Fixed the guard.
• Edit cancel leaves stale content: When cancelling a message edit, the composer retained the edit content instead of restoring the pre-edit draft. Added preEditContentRef to save/restore the draft around edit mode transitions.

Test plan

• Edit a message, click Cancel — composer should restore whatever was typed before (or be empty)
• Edit a message, press Escape — same behavior
• Edit a message, save — composer clears correctly, no stale content
• Type a draft, enter edit mode, cancel — draft is preserved
• Send a message containing only #channel-name — renders as a link
• Send a message with inline code and code blocks — backticks not double-escaped
• Edit a message in a thread — edit stays scoped to thread composer, not main
• Edit a message in main timeline while thread is open — edit stays in main composer

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