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/lib/api/providers.ts

@@ -103,6 +103,10 @@ export const providersApi = {
     return await invoke("ensure_claude_desktop_official_provider");
   },
 
+  async ensureCodexOfficialProvider(): Promise<boolean> {
+    return await invoke("ensure_codex_official_provider");
+  },
+
   async getClaudeDesktopStatus(): Promise<ClaudeDesktopStatus> {
     return await invoke("get_claude_desktop_status");
   },