fix: 修复 SLS 实例删除残留外部数据与重装共用文件夹

删除实例残留外部数据：
- remove_instance 原先只在 game_dir_override 指向 versions/<name>
  隔离目录时才删除外部目录；SLS（hosted）实例使用版本共享布局
  （外部目录直接作为游戏根），删除时落到托管目录分支，外部 mods/
  存档/配置残留。
- 现在 game_dir_override 只要指向实例独占目录（非共享 .minecraft
  根）就一并删除；新增 is_shared_minecraft_root 判据：含
  libraries/ 或 assets/ 的目录视为共享游戏根，删除实例时保留。

重装共用同一文件夹：
- hosted::create 生成 game_dir_override 时直接拼接
  `<root>/<pack name>`，无冲突处理；同一整合包安装两次会指向同一
  目录，两个实例共用一份游戏数据。
- 新增 unique_game_dir：目标目录已存在时依次尝试 `<name> (1)`、
  `<name> (2)` …，与 create_instance::resolve_instance_path 的
  实例目录去重逻辑保持一致。

packages/app-lib/src/state/instances/commands/remove_instance.rs

@@ -32,12 +32,17 @@ pub(crate) async fn remove_instance(
         .game_dir_override
         .as_deref()
         .map(PathBuf::from)
-        .filter(|path| is_version_isolated_game_dir(path))
+        .filter(|path| {
+            // Delete the external game directory when the instance owns it.
+            // A version-isolated `versions/<name>` folder is obviously
+            // exclusive; a plain `<root>/<pack name>` folder is also owned by
+            // this instance. A shared `.minecraft` root, however, holds the
+            // game's libraries/assets and must never be deleted with one
+            // instance.
+            is_version_isolated_game_dir(path)
+                || !is_shared_minecraft_root(path)
+        })
     {
-        // New instances created against a configured `.minecraft` root use
-        // a private `versions/<name>` directory. Remove that external
-        // directory when the instance is deleted, while preserving shared
-        // (non-isolated) overrides for backwards compatibility.
         game_dir_override
     } else {
         state.directories.instances_dir().join(&instance.path)
@@ -69,3 +74,12 @@ fn is_version_isolated_game_dir(path: &Path) -> bool {
         .and_then(|name| name.to_str())
         == Some("versions")
 }
+
+/// Heuristic: a shared `.minecraft` root holds the game's libraries and
+/// assets, which must survive the removal of any single instance that points
+/// at it. Instance-owned external folders (e.g. a hosted modpack's
+/// `<root>/<pack name>` directory) contain only mods/saves/config and no such
+/// shared game body.
+fn is_shared_minecraft_root(path: &Path) -> bool {
+    path.join("libraries").is_dir() || path.join("assets").is_dir()
+}