Announce new chat messages to screen readers #64

Screen reader users don't hear new replies arrive. The typing indicator and the message box also have no accessible text. Each fix here is small, mostly a single attribute.

Where to look

• packages/widget/src/dom/messages.ts (the .cc-messages container in the constructor, and showTyping())
• packages/widget/src/dom/input.ts (the textarea)
• packages/widget/src/i18n.ts (typingIndicator exists but is never used)
• packages/widget/tests/dom/messages.test.ts, and packages/widget/tests/dom/input.test.ts for the textarea

Acceptance criteria

• The messages container has role="log" and aria-live="polite", so new replies are announced
• The typing indicator has accessible text from typingIndicator (visually hidden text or aria-label), and the three dots are aria-hidden="true"
• The textarea has an aria-label (today it only has a placeholder; reusing inputPlaceholder is fine)
• Screen readers don't re-read the whole reply on every chunk while it streams (for example, set aria-busy="true" on the streaming message and remove it when complete)
• Tests for the new attributes
• Changeset for @chatcops/widget (patch)

Want to work on this? Comment below and I'll assign it to you. Please take one issue at a time, branch off dev, and open your PR against dev with Closes #<this issue number>. Setup notes (pnpm, build before test) are in CONTRIBUTING.md.