Bug: PartnerManager stores partner by player name instead of UUID

[dbaccello]

Description

In PartnerManager, partner entries are saved as player names (String) instead of UUIDs.

Current behavior

• addPartner(UUID uuid, String partner) stores the partner name
• isPartner(UUID uuid, String partner) checks the partner name
• partners.json stores names instead of UUIDs

Problem

This is fragile because player names are not permanent. If a player changes name, the partner association can break. If name reuse occurs, the wrong player could be treated as a partner.

Expected behavior

Partner associations should use UUIDs as the stable identifier.

Affected file

paper/src/main/java/de/clickism/clickvillagers/villager/PartnerManager.java