fix(codex): move common-config TOML merge off smol-toml to backend toml_edit

updateTomlCommonConfigSnippet re-serialized the whole document through
smol-toml (parse -> deepMerge -> stringify): comments dropped, keys
reordered, and empty parent table headers synthesized -- the
long-standing "config.toml keeps getting reordered" symptom (audit C5,
introduced in 083e48bf).

Replace it with a backend command backed by the same
merge_toml_table_like / remove_toml_table_like used when writing live
configs, so the form preview and the live write share one merge
semantic and user formatting survives edit-time merges. The frontend
sync helper is deleted outright to keep the pattern from coming back.

Making the form operations async exposes them to interleaving, so a
result is discarded unless it is still current when it lands:

- a per-hook sequence number (last operation wins) covers rapid
  toggle/save races where an earlier merge resolves after a later
  removal and would flip the switch back;
- a config-baseline check covers the user hand-editing the TOML while
  a merge is in flight -- the stale result must not clobber their edit.
  The checkbox state self-heals via the existing inference effect.

Regression tests pin both by resolving a suspended merge after a newer
operation / an external edit, plus backend tests locking comment and
key-order preservation, scalar override, and value-matched removal.

src-tauri/src/commands/config.rs

@@ -275,6 +275,23 @@ pub async fn get_common_config_snippet(
         .map_err(|e| e.to_string())
 }
 
+/// 对前端编辑器里的 config.toml 文本做通用配置片段的合并/剥离。
+/// 放后端是为了走 toml_edit（保注释、保键序）；前端 smol-toml 的
+/// 整文档重序列化会破坏用户手写格式。
+#[tauri::command]
+pub async fn update_toml_common_config_snippet(
+    config_toml: String,
+    snippet_toml: String,
+    enabled: bool,
+) -> Result<String, String> {
+    crate::services::provider::update_toml_common_config_snippet(
+        &config_toml,
+        &snippet_toml,
+        enabled,
+    )
+    .map_err(|e| e.to_string())
+}
+
 #[tauri::command]
 pub async fn set_common_config_snippet(
     app_type: String,