feat(chat): add @global world chat command

[DorWeid]

Description

• add a rank-0 @global <message> player command
• broadcast a type-6 server notice to every online character in the sender's world
• preserve the sender name and original message casing in [Global] Name: message
• log successful messages as Global chat and reject blank messages with usage guidance

Verification

• ./mvnw.cmd -q -Dtest=GlobalCommandTest test
• ./mvnw.cmd -q test — 1,888 tests, 0 failures/errors/skips
• ./mvnw.cmd -q -DskipTests package

Live multi-character verification was not run locally; the unit test decodes and asserts the exact SERVERMESSAGE packet sent to the world broadcaster.

Checklist before requesting a review

• I have performed a self-review of my code
• I have tested my changes
• I have added unit tests that prove my changes work

Screenshots

Not applicable.