fix(studio): prevent SourceEditor recreation on every keystroke

[Jefsky]

Summary

The SourceEditor component was destroying and recreating the CodeMirror EditorView on every keystroke, because mountEditor was used as both a callback ref and a useCallback with content in its dependency array.

What changed

1. Removed content from the mountEditor dependency array
2. Added a contentRef to pass initial content without subscribing to changes
3. Added a useEffect that syncs external content changes (file switches, server refreshes) into the existing editor via dispatch() without recreating it

Result: cursor position, undo history, and focus are preserved across keystrokes.

Closes #624

[miguel-heygen]

Closing in favor of #624 which has the same fix.