Update Codex login success page UX (#20136)

## Summary

update the local login success page to match the Codex desktop auth UX
use theme-aware colors and an inline 20px Codex mark
keep the actual localhost success page aligned with the browser auth UX
PR

## Tests

<img width="1728" height="1117" alt="Screenshot 2026-04-29 at 12 00
34 PM"
src="https://github.com/user-attachments/assets/76a40c3f-07c3-452c-97da-e7c43717cd2c"
/>

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -6729,6 +6729,9 @@
         },
         {
           "properties": {
+            "codexStreamlinedLogin": {
+              "type": "boolean"
+            },
             "type": {
               "enum": [
                 "chatgpt"