fix(settings): disable confirm-code buttons while throttled

[vbudhram]

Because

• On a THROTTLED (errno 114) response from confirm-code pages, Submit and "Email new code" stayed clickable. Users clicked 10-12 times, which inflated error-rate metrics.
• The lockout window was not announced to screen readers.

This pull request

• Adds a useThrottleCountdown hook that counts down retryAfter seconds, or stays throttled when only retryAfterLocalized is present.
• Adds throttleSecondsRemaining and throttleLocalizedFallback props to FormVerifyCode. The submit button is disabled while throttled, and an aria-live="polite" region shows "Try again in N seconds".
• Hooks the throttle state into SigninTokenCode, SigninPasswordlessCode, and ConfirmSignupCode on their THROTTLED branches, and disables each page's "Email new code" button for the same window.
• Adds FTL strings form-verify-code-throttled-countdown and form-verify-code-throttled-generic.
• Adds unit tests for the hook and for the disabled-state assertions on each consumer page.

Issue that this pull request solves

Closes: https://mozilla-hub.atlassian.net/browse/FXA-13742

Checklist

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• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.