feat(codex): expose official routing and restore the built-in provider

Let users switch between the built-in OpenAI provider and third-party Codex providers directly from the provider panel while takeover mode remains active.

Centralize the frontend capability predicate so only the fixed codex-official seed receives native-login routing support, and keep copied UUID-based official entries clearly marked as unsupported.

Add an idempotent backend command that recreates the deleted official seed, wire it into the add-provider flow, refresh localized guidance, and add mutation and provider-action regression coverage.

src-tauri/src/lib.rs

@@ -1199,6 +1199,7 @@ pub fn run() {
             commands::get_claude_desktop_default_routes,
             commands::import_claude_desktop_providers_from_claude,
             commands::ensure_claude_desktop_official_provider,
+            commands::ensure_codex_official_provider,
             commands::get_claude_config_status,
             commands::get_config_status,
             commands::get_claude_code_config_path,