fix: prevent SourceEditor recreation on every keystroke

[hobostay]

Summary

• Fix SourceEditor being destroyed and recreated on every content change, losing cursor position, undo history, and focus
• The mountEditor callback had content in its dependency array and was used as a React ref callback — every keystroke changed content, giving mountEditor a new identity and triggering full editor recreation

Details

Affected file: packages/studio/src/components/editor/SourceEditor.tsx

The fix:

1. Removes content from the mountEditor dependency array — the editor is now only recreated when filePath, language, or readOnly change
2. Uses a contentRef to pass initial content to the editor without subscribing to changes
3. Adds a useEffect that syncs external content changes (file switches, server refreshes) into the existing editor via view.dispatch(), without recreating it

Test plan

• Open a file in the source editor and type — verify cursor position is maintained between keystrokes
• Verify undo/redo (Ctrl+Z/Ctrl+Y) still works
• Switch files and verify the editor content updates correctly
• Verify syntax highlighting works after file switch

🤖 Generated with Claude Code

[jrusso1020]

Verified — mountEditor is used as a ref callback (<div ref={mountEditor}>) with content in its dep array, so every keystroke gives it a new identity. React then invokes the old callback with null (destroying the editor) and the new one with the node (rebuilding it), losing cursor/undo/focus. The fix is the canonical pattern: drop content from the callback deps, seed via a ref, and sync external changes through a separate useEffect that dispatches a single replace transaction. The current !== content guard inside the effect correctly avoids a feedback loop with the updateListener (which would otherwise re-fire onChange after every external sync). LGTM.