Clear RoomCodeCopyButton Fast Refresh warnings

[christopherjnelson]

Summary

• Moved copyRoomCode, scheduleRoomCodeCopyReset, ROOM_CODE_COPY_FEEDBACK_MS, and RoomCodeCopyFeedback into roomCodeCopy.ts.
• RoomCodeCopyButton.tsx now exports only React components.
• Updated production and test imports without changing behavior.
• Added no ESLint suppression or configuration exception.

Verification

• Targeted RoomCodeCopyButton tests passed.
• Full Vitest remains 708 passed and 3 skipped.
• Formatting, build, and diff checks passed.
• Lint completes with zero errors and zero warnings.
• No snapshots, CI, scripts, package metadata, reports, or rendered behavior changed.