Fix user-message copy action on mobile

[AnthonyRonning]

Summary

• keep user-message actions below and right-aligned on mobile layouts
• preserve the compact side-mounted hover treatment at the md breakpoint and wider
• update the shared ChatTurn regression test for both responsive layouts

Validation

• bun --no-env-file test (739 passed)
• bun run format:check
• bun run lint (0 errors; 13 existing warnings)
• bun run typecheck
• bun run build
• git diff --check

[cloudflare-workers-and-pages]

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