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/types.ts

@@ -126,6 +126,8 @@ export interface ProviderProxyConfig {
 export interface ProviderMeta {
   // 自定义端点：以 URL 为键，值为端点信息
   custom_endpoints?: Record<string, CustomEndpoint>;
+  // 是否在切换/同步到 live 时应用通用配置片段
+  commonConfigEnabled?: boolean;
   // 用量查询脚本配置
   usage_script?: UsageScript;
   // 请求地址管理：测速后自动选择最佳端点