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/lib/api/config.ts

@@ -48,6 +48,29 @@ export async function setCommonConfigSnippet(
   return invoke("set_common_config_snippet", { appType, snippet });
 }
 
+/**
+ * 对编辑器里的 config.toml 文本做通用配置片段的合并/剥离
+ *
+ * 合并/剥离在后端用 toml_edit 完成（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，禁止在前端做这类结构化改写。
+ *
+ * @param configToml - 编辑器当前的 config.toml 文本
+ * @param snippetToml - 通用配置片段（TOML 字符串）
+ * @param enabled - true 合并片段，false 按值匹配剥离片段
+ * @returns 更新后的 config.toml 文本
+ */
+export async function updateTomlCommonConfigSnippet(
+  configToml: string,
+  snippetToml: string,
+  enabled: boolean,
+): Promise<string> {
+  return invoke<string>("update_toml_common_config_snippet", {
+    configToml,
+    snippetToml,
+    enabled,
+  });
+}
+
 /**
  * 提取通用配置片段
  *