feat: apply common config as runtime overlay instead of materialized merge

Common config snippets are now dynamically overlaid when writing live
files, rather than being pre-merged into provider snapshots at edit time.
This ensures that updating a snippet immediately takes effect for the
current provider and automatically propagates to other providers on
their next switch.

Key changes:
- Add write_live_with_common_config() overlay pipeline
- Strip common config from live before backfilling provider snapshots
- Normalize provider snapshots on save to keep them snippet-free
- Add explicit commonConfigEnabled flag in ProviderMeta (Option<bool>)
- Migrate legacy providers on snippet save (infer flag from subset check)
- Add Codex TOML snippet validation in set_common_config_snippet
- Stabilize onConfigChange callbacks with useCallback in ProviderForm

src-tauri/src/provider.rs

@@ -197,6 +197,9 @@ pub struct ProviderMeta {
     /// 自定义端点列表（按 URL 去重存储）
     #[serde(default, skip_serializing_if = "HashMap::is_empty")]
     pub custom_endpoints: HashMap<String, crate::settings::CustomEndpoint>,
+    /// 是否在写入 live 时应用通用配置片段
+    #[serde(rename = "commonConfigEnabled", skip_serializing_if = "Option::is_none")]
+    pub common_config_enabled: Option<bool>,
     /// 用量查询脚本配置
     #[serde(skip_serializing_if = "Option::is_none")]
     pub usage_script: Option<UsageScript>,