Build ts3j from source as part of the client build

The client resolved ts3j 1.0.3 from ~/.m2, so a machine that pulled new
ts3j sources without re-running mvn install silently shipped the old
library - here one without the packet generation fix, which fails
connects to busy servers with "mac check in EAX failed".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ts3-client/README.md

@@ -189,11 +189,11 @@ frontend supplies its own UI and audio backend while reusing `core` unchanged.
   - macOS: `brew install opus`
 
 ## Building
-The client depends on `ts3j`, so install that into your local Maven repo first:
+The build compiles `ts3j` from `../ts3j` alongside the client, so make sure the submodule is
+checked out (`git submodule update --init`):
 
 ```bash
-cd ../ts3j && mvn -DskipTests install
-cd ../ts3-client && mvn -DskipTests package
+mvn -DskipTests package
 ```
 
 This produces a runnable fat-jar at `swing/target/ts3-client.jar`.