feat(chat): add copy-to-clipboard button to message bubbles

[JnyRoad]

Summary

Chat messages had no way to copy their text, unlike the copy-to-clipboard affordance under each message in Claude/ChatGPT-style chat UIs. This adds that button to both user and assistant messages in the chat page.

Changes

• Add a copy button to MessageBubble: merged into the existing feedback row (next to thumbs up/down) for assistant replies, and a standalone row for user messages. Reuses the existing copyTextToClipboard helper and notify toast for success/failure feedback.
• Hide the button while an assistant reply is still streaming, since the displayed text can change before an in-flight clipboard write resolves (would otherwise show a stale "copied" confirmation against newer text).
• Guard state updates after the async clipboard write with a mounted ref, so a write that resolves after the component unmounts (e.g. switching sessions mid-copy) doesn't touch state or leave a dangling timer. The ref is reset on effect setup, not just cleared on cleanup, so it survives React StrictMode's dev-mode double mount/unmount cycle.
• Reset the "copied" confirmation timer via a ref on every click, so a repeated click before the first 1.5s window expires gets its own full confirmation window instead of being cut short by the earlier timer.

Testing

• cd frontend-enterprise && npx tsc -b — clean
• cd frontend-enterprise && npx vitest run src/pages/chat — 64 passed

Risks

No obvious risks. Change is scoped to a single component (MessageBubble.tsx) and its test file; no API, schema, or config changes.