Fixes AWiki registration retargeting and registration error feedback.
- Clears stale pending registration material after an OTP is successfully requested for a different Handle or phone number.
- Preserves recoverable pending registration state when retrying the same registration target.
- Classifies Handle occupancy and existing DID responses before generic RPC conflict handling.
- Shows an actionable Handle-already-exists message instead of a generic conflict.
- Shows a dedicated Chinese verification-code rate-limit message while preserving the registration form draft.
- Adds SDK and browser-controller regression coverage for retargeting, conflict classification, and user-facing failures.