Fix Markdown copy buttons and scrollbar dragging

[Anton-Horn]

Focus updates could replace Markdown elements between mouse-down and click, interrupting code-copy buttons and scrollbar drags. Keep the Markdown renderers stable so controls retain their DOM identity, including code blocks inside lists and quotes.

Code copying now uses the existing native clipboard bridge, shows success only after the write completes, and provides retry feedback on failure.

Validation:

• 60 focused unit tests passed, including DOM/focus preservation and clipboard success/failure coverage.
• 3 Electron tests passed for copying and horizontal scrollbar dragging in plain blocks, lists, and quotes at 65%, 100%, and 180% zoom.
• Typecheck and production build passed. Targeted lint reported no errors (two existing EditorPanel warnings).

Only the five files implementing and testing this Markdown fix are included.