fix(widget): announce new messages and the typing indicator to screen readers

[prakashNalla]

What does this change?

The messages container now has role="log" and aria-live="polite", so new replies are read out. The typing indicator gets the typingIndicator locale text in a visually hidden span (new .cc-sr-only class in widget.css) and its three dots are aria-hidden. The textarea gets an aria-label from the placeholder, and a reply has aria-busy="true" while it streams, which is removed once it's complete, so it isn't re-read on every chunk.

Closes #64

How I tested this

• pnpm install --frozen-lockfile, pnpm -r build, pnpm -r typecheck, pnpm test (29 files / 265 tests pass) and pnpm changeset status, on Node v22.23.3 with pnpm 9.15.0
• Added 4 tests (the log attributes, the typing text and hidden dots, aria-busy while streaming, the textarea label). All 4 fail without the change.
• Opened dev.html against a small local server that streams a reply word by word, and read the accessibility tree in headless Chrome (puppeteer page.accessibility.snapshot()):
  • textarea: textbox "Type a message..."
  • while waiting: log containing "Typing...", dots aria-hidden="true"
  • mid-stream the reply has aria-busy="true", after the last chunk it's gone, and the log holds the full reply

AI assistance

Claude helped me w/ the understanding of the tests.

Checklist

• This PR targets dev
• I was assigned the linked issue
• pnpm -r build, pnpm test and pnpm -r typecheck pass locally
• Tests added or updated where it makes sense
• For a bug fix: the new test fails without my change
• Changeset added (pnpm changeset) if this changes core, widget or server
• Docs updated if behavior or options changed