Delegate post-merge registration routing to the KYC flow

[TaprootFreak]

Summary

Fixes the post-account-merge "Wallet registration not complete" error (red banner) in the RealUnit app, the CONTRIBUTING-aligned way.

When a new wallet signs up with an email that already belongs to an existing DFX account (account merge) and that account has no prior RealUnit registration, GET /v1/realunit/registration returns state=NewRegistration. The email-verification step, however, called registerWallet (POST /register/wallet) unconditionally — that endpoint only adds a wallet to an existing registration, so the API returns 400 "No RealUnit registration found".

Root cause — runtime-confirmed (PROD App Insights)

• GET /v1/realunit/registration → 200 (state=NewRegistration, userData present from existing KYC)
• POST /v1/realunit/register/wallet → 400 {"message":"No RealUnit registration found"} (×4 retries)
• recovery: the user reached the KYC registration form → POST /v1/realunit/register/complete → 201
• DB: the merged account had no RealUnitRegistration step until register/complete created it. 7-day breadth: 7× this 400 (affects every merge into a DFX account without a prior RealUnit registration).

Fix (app-only) — single source of registration routing

The email-verification flow is reduced to its actual job: confirm the merge (detect the JWT account change) and hand back to the KYC flow. KycCubit is now the only place that interprets the registration state and routes it (addWallet → link wallet, NewRegistration → full registration form, AlreadyRegistered → forward) — per CONTRIBUTING.md "API as Decision Authority". This removes the duplicated, unconditional register/wallet call.

Dead code removed accordingly: _completeRegistration, _mergeDetected, the RealUnitRegistrationService dependency, the KycEmailVerificationRegistrationFailure state, and the now-unused i18n key registerEmailVerificationRegistrationFailed (de + en).

Tests

• email cubit (simplified): same account → Failure (link not visited); changed account → Success (merge confirmed; no registration here); retry (Failure → Success).
• kyc_step_states_test updated for the removed state.

Test plan

• flutter analyze clean
• flutter test --exclude-tags golden — full suite passes (2312)
• Coverage Floor Gate replicated locally — scoped lines 100.0% (floor 100)
• DEV end-to-end: merge into a DFX account without RealUnit registration → no red error → lands on the registration form → register/complete

Supersedes #711 (minimal variant) and the earlier incorrect MergeProcessing PRs DFXswiss/api#3848 + #709.