ui-svelte: add copy button to markdown code blocks (#537)

Add a copy-to-clipboard button that appears on hover for each code block
rendered in the chat interface assistant messages.

- Svelte action `codeBlockCopy` injects a button into every `<pre>`
element
- MutationObserver reattaches buttons as streaming content arrives
- Button shows a check icon for 2 seconds after a successful copy
- Uses clipboard API with execCommand fallback for non-secure contexts
- CSS hides button by default and reveals it on pre:hover

https://claude.ai/code/session_01PTA5ao5YQuFAS6a9juLeZW

---------

Co-authored-by: Claude <noreply@anthropic.com>

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