feat(code): add copy source button to markdown viewer

[dmarticus]

Problem

When viewing a markdown file in PostHog Code's split view, there's a "View rendered" toggle but no way to grab the raw source. If you want to copy the markdown (e.g. to paste into Slack, an issue, or another doc), you have to switch to source view, select-all, then copy.

Changes

Added a Copy icon button next to the existing rendered/source toggle in the markdown viewer header. Clicking it copies the file's source contents to the clipboard and swaps the icon to a checkmark for 2s — same pattern as CodeBlock.tsx.

Demo

https://screen.studio/share/gxuVthEc

How did you test this?

• Pre-commit hook ran biome + typecheck — both pass.
• Have not yet manually verified in the running app (no dev server spun up here).

Publish to changelog?

no