refactor(01): align scaffold with Mythlane conventions (Kotlin DSL, CamelCase manifest, pinned server version)

- Convert build.gradle/settings.gradle to Kotlin DSL matching VotePipe + myth_*
- Manifest keys: lowercase -> CamelCase (Group/Name/Version/Main/ServerVersion/...)
  matching production Mythlane plugins (empirical authority)
- gradle.properties adopts pluginGroup/pluginVersion/pluginDescription/hytaleServerVersion
- Template manifest via processResources.expand(${version}, ${description}, ${hytaleServerVersion})
- Add baseline deps: gson (relocated), jetbrains annotations, JUnit 5
- shadowJar archiveBaseName=hytale-gravity-flip

Rebuild: BUILD SUCCESSFUL, fat JAR regenerated with CamelCase manifest.

src/main/resources/manifest.json

@@ -1,11 +1,20 @@
 {
-  "group": "com.mythlane",
-  "name": "hytale-gravity-flip",
-  "version": "0.1.0",
-  "serverVersion": "2026.03.26-89796e57b",
-  "description": "Inverted gravity zones defined via an in-game wand.",
-  "authors": [
-    { "name": "Mythlane" }
+  "Group": "Mythlane",
+  "Name": "GravityFlip",
+  "Version": "${version}",
+  "Description": "${description}",
+  "Authors": [
+    {
+      "Name": "Mythlane Team",
+      "Email": "contact@mythlane.com",
+      "Url": "https://mythlane.com"
+    }
   ],
-  "main": "com.mythlane.gravityflip.GravityFlipPlugin"
+  "Website": "https://mythlane.com",
+  "Main": "com.mythlane.gravityflip.GravityFlipPlugin",
+  "ServerVersion": "${hytaleServerVersion}",
+  "Dependencies": {},
+  "OptionalDependencies": {},
+  "DisabledByDefault": false,
+  "IncludesAssetPack": false
 }