Fix server-name caching and include server name in email body

[dmccoystephenson]

Summary

• Cache server-name at load time (Herald.java): server-name was being read from config on every PlayerJoinEvent. It is now read once in loadConfiguration() and stored as a serverName field, matching how Discord and email settings are handled.
• Include server name in email body (EmailNotifier.java): The email body said "Steve has joined the server at <date>" — it never mentioned which server. It now reads "Steve has joined MySurvivalServer at <date>", consistent with the email subject.
• Updated tests (EmailNotifierTest.java): Assertions that checked the old body format are updated to verify the server name is present.

Test plan

• ./gradlew build -x test succeeds
• ./gradlew clean test passes all tests
• Email body format verified to include server name via test capture

Closes #13
Closes #14

[Copilot]

Pull request overview

This pull request addresses two related notification-quality issues by (1) caching the configured server name once at startup and (2) ensuring the server name is included in the email body content, aligning the body with the subject and improving clarity for multi-server setups.

Changes:

• Cache server-name during Herald.loadConfiguration() and reuse it for join notifications.
• Update EmailNotifier.notifyPlayerJoin() so the email body includes the serverName.
• Update EmailNotifierTest assertions to verify the server name appears in the email body.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/main/java/com/dansplugins/herald/Herald.java	Caches serverName at configuration load time and uses it when sending join notifications.
src/main/java/com/dansplugins/herald/EmailNotifier.java	Adds serverName into the email body format for join notifications.
src/test/java/com/dansplugins/herald/EmailNotifierTest.java	Updates tests to assert the new email body format includes the server name.

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