feat(codex): add unified session history toggle for official providers

Codex buckets resume history by the model_provider id recorded in each
session: official runs (no key, built-in "openai") and cc-switch
third-party runs (shared "custom") are mutually invisible in the resume
picker. Add an opt-in setting that runs official providers under the
shared "custom" id so future official sessions land in the same history
bucket as third-party ones. Forward-only by design: existing sessions
are not migrated.

When enabled, official live config.toml gets model_provider = "custom"
plus a [model_providers.custom] entry that mirrors the built-in openai
provider (requires_openai_auth routes auth to the ChatGPT login in
auth.json, name "OpenAI" keeps is_openai() feature gates, explicit
supports_websockets/wire_api restore built-in defaults). auth.json is
untouched.

Key invariants:
- Injection lives only in the live config: switch-away backfill strips
  the exact injected shape, so stored provider configs stay clean and
  turning the toggle off fully reverts on the next write.
- Toggle changes apply immediately via a takeover-aware reapply: when
  the proxy owns the live config (backup/placeholder present), only the
  live backup is updated, mirroring the provider-switch path.
- The takeover backup path runs the same injection so a takeover
  release restores a config that still carries the unified routing.
- Injection refuses to activate a foreign [model_providers.custom]
  table (e.g. stale entry with a third-party base_url) to avoid routing
  ChatGPT OAuth traffic to an unknown backend.

The toggle lives under Settings → Codex App Enhancements; the
description warns that resuming old sessions across providers may fail
because encrypted_content reasoning only decrypts on the backend that
created it (upstream treats cross-provider resume as unsupported).

src-tauri/src/commands/settings.rs

@@ -65,10 +65,25 @@ pub async fn get_settings() -> Result<crate::settings::AppSettings, String> {
 
 /// 保存设置
 #[tauri::command]
-pub async fn save_settings(settings: crate::settings::AppSettings) -> Result<bool, String> {
+pub async fn save_settings(
+    state: tauri::State<'_, crate::store::AppState>,
+    settings: crate::settings::AppSettings,
+) -> Result<bool, String> {
     let existing = crate::settings::get_settings();
     let merged = merge_settings_for_save(settings, &existing);
+    let unify_codex_changed =
+        merged.unify_codex_session_history != existing.unify_codex_session_history;
     crate::settings::update_settings(merged).map_err(|e| e.to_string())?;
+
+    // 统一会话开关变更时立即重写当前官方 Codex 供应商的 live 配置，
+    // 不必等下一次切换才生效。
+    if unify_codex_changed {
+        if let Err(err) =
+            crate::services::provider::reapply_current_codex_official_live(state.inner())
+        {
+            log::warn!("统一 Codex 会话历史开关变更后重写 live 配置失败: {err}");
+        }
+    }
     Ok(true)
 }