refactor: drop Joda-Time (-> java.time) and make protocol charsets explicit

Modernization following the dependency upgrades:
- Joda-Time was used in exactly one place (ModToolSanctionInfoComposer, to
  subtract probation days from a Date). Migrated to java.time
  (Instant/ZoneId.systemDefault, calendar-accurate like the old Joda call) and
  removed the joda-time dependency entirely — confirmed gone from the shaded jar.
- Make string<->bytes conversions explicitly UTF-8 instead of relying on the
  platform default. Most importantly the wire codec (ClientMessage.readString /
  ServerMessage.appendString) — both sides now pinned to UTF-8 so international
  characters are robust regardless of -Dfile.encoding. Also RCONServerHandler,
  PluginManager and the WS origin-forbidden response.

Verified: clean compile, 15/15 tests, shaded jar.

Emulator/pom.xml

@@ -163,13 +163,6 @@
             <version>2.4.3</version>
         </dependency>
 
-        <!-- Joda Time -->
-        <dependency>
-            <groupId>joda-time</groupId>
-            <artifactId>joda-time</artifactId>
-            <version>2.14.2</version>
-        </dependency>
-
         <!-- jBCrypt � used by the built-in /api/auth/* HTTP login handler
              to verify Laravel-style $2y$ BCrypt hashes from users.password -->
         <dependency>