fix: pass erpParty when changing account level to Pro/Merchant

[islandbitcoin]

Problem

After #52 surfaced real backend errors, Change Level in Account Hub fails with:

erpParty is required for level 2 and 3 accounts

...even for a THREE → TWO downgrade of an account that already has an ERP party (e.g. the account's ERP Party is shown right there in the Overview). So Pro/Merchant re-leveling is currently impossible from this button.

Root cause

Flash's accountUpdateLevel mutation requires an erpParty whenever the resulting level is TWO (Pro) or THREE (Merchant). The plumbing was half-built:

• GraphQLClient.update_account_level(uid, level, erp_party=None) already accepts it.
• But admin_api.update_account_level(uid, level) didn't take or forward it, and the change_level dialog sent only {uid, level}.

The account object already carries erpParty (rendered in the Overview) — it just wasn't passed back into the mutation. The correct precedent already exists in approve_upgrade_request (admin_api.py), which supplies erp_party for TWO/THREE.

Fix

• admin_api.update_account_level — accept erp_party=None and forward it to the client.
• change_level (JS) — when the target level is TWO/THREE, pass the account's existing erpParty. If such an account has no party, block with an actionable message pointing to the upgrade-request approval flow (the only path that mints the ERP Customer/Address/Bank records) instead of fabricating a dangling party.

Downgrades to ZERO/ONE and re-leveling of accounts that already have a party work; the mint-a-new-party case is intentionally still routed through upgrade-request approval.

Verification

• node --check on the page JS and py_compile on admin_api.py both pass.
• The prior error was "erpParty is required" (not "account not found"), which confirms uid already resolves and erpParty was the only missing input.
• End-to-end check on TEST: select a Merchant (level THREE) account with an ERP party → Change Level → Pro → expect success, and the level badge updates.

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