feat: upgrade config and language files in place - #96

Summary

• merge missing settings from the bundled config.yml without replacing existing values
• merge missing translation entries from the bundled locale file without replacing translations
• update old config and language versions in place instead of renaming the files and generating a blank replacement
• add recursive YAML merge tests

Behavior

When a user-managed YAML file is loaded, each missing nested key is filled from the corresponding resource bundled in the plugin JAR. Existing values are preserved, including false, 0, lists, and custom translated strings. A custom locale without a bundled resource uses en_US.yml as its default source. Newer file versions are preserved and only logged so a downgraded plugin does not rewrite them.

Verification

• mvn -B test (6 tests, 0 failures)
• mvn -B package

This addresses automatic upgrade of user configuration and language files while retaining administrator customizations.