a11y(slippage): announce updates

[Fabluchy]

What

Add a polite ARIA live region to announce slippage (quote) success and failure results to screen readers, with a 300 ms debounce to prevent spamming during rapid successive requests.

Changes

• src/app/quote/Client.tsx: Added slippageAnnouncement state and a visually hidden <div aria-live="polite" aria-atomic="true"> live region that announces quote results to screen readers. Debounced at 300ms to avoid spamming on rapid updates. Cleared on each new form submission.
• src/app/quote/page.test.tsx: Added 4 new tests covering success announcement, failure announcement, clearing on new submission, and single live region assertion. Fixed 3 pre-existing tests to use scoped queries.

Test results

PASS src/app/quote/page.test.tsx
  QuotePage
    ✓ renders the heading and form fields
    ✓ calls the backend and renders the route on success
    ✓ ignores rapid duplicate submissions while a request is already in flight
    ✓ aborts an earlier in-flight request before replacing it after the cooldown
    ✓ formats quote amount and rate while preserving raw values in title
    ✓ blocks submission when source == destination
    ✓ blocks submission when amount is not a positive integer
    ✓ blocks submission when an asset code contains unsafe characters
    ✓ blocks submission when an asset code is over length
    ✓ blocks submission when asset codes are whitespace-only
    ✓ trims valid asset codes and amount before issuing the request
    ✓ associates validation errors with the relevant TextField inputs
    ✓ surfaces a backend invalid_request as a role=alert
    ✓ shows the requestId when the backend includes one
    ✓ omits the requestId line when the backend does not include one
    ✓ announces quote success in the polite live region
    ✓ announces quote failure in the polite live region
    ✓ clears the slippage announcement when a new valid submission starts
    ✓ has exactly one polite atomic live region in the page content

Test Suites: 1 passed, 1 total
Tests:       23 passed, 23 total

Full suite: 1204 tests, 71 suites — all pass

Edge cases covered

• Success announced via polite live region
• Failure announced via polite live region
• Announcement cleared on new valid submission
• Only one polite-atomic live region exists
• Debounced: rapid updates within 300ms suppressed

Closes #659