feat(chat): copy the AI answer from the message action bar

[squarezw]

Adds a copy button next to the existing export / like / dislike actions. It copies the answer's markdown — the same content the PDF export uses. Clicking it swaps the icon to a check for two seconds and raises a toast.

The three i18n keys already existed in chat.json / common.json and had no callers: chat.copyAnswer, common.copied, common.copyFailed. This change just puts them to use — no new strings.

Why this isn't a one-line navigator.clipboard call

The async Clipboard API only exists in a secure context. Customer deployments are served over plain HTTP on an internal address (http://192.168.80.3:3001), where navigator.clipboard is undefined and calling it throws a TypeError.

That is the worst kind of failure: the button reacts, nothing is reported, and the clipboard still holds whatever was there before — so the user pastes stale content believing the copy worked.

lib/clipboard.ts therefore falls back to document.execCommand("copy") — deprecated, but still the only path available outside a secure context — and, crucially, returns false when it genuinely cannot copy, so the caller surfaces an error instead of pretending.

Two details that decide whether the fallback works at all:

• The textarea is moved off-screen rather than display: none. A hidden element cannot be selected, so the copy fails silently.
• Focus is restored afterwards, or the user loses their place in whatever input they were typing in.

The secure-context check also guards a subtler case: some browsers keep the clipboard object on HTTP but reject every call. Without the check, each copy would produce a rejected promise and a console error before falling back.

Verification

• 268 tests pass (10 new)
• 7 mutations, all red: no fallback / no secure-context check / fallback lies about success / textarea left in the DOM / focus not restored / display:none instead of off-screen / empty content still copied
• One mutation initially survived — "no secure-context check". That test only asserted the end result, which is identical down either path. It now asserts writeText was not called.
• tsc: non-TS5097 errors 112, unchanged from baseline. The single TS5097 is the .ts import extension that every other test file in this repo also carries.

Not verified

Rendering was not checked in a browser: the local dev server on :3000 runs the private repo, and the chat view requires an authenticated session.

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