Add hidden synthetic retry turn and UI to retry unrecovered activity failures: compact collapsed trace, retry button with spinner, and enter/exit animations (styles). Wire retry flow through ChatView -> ChatFeed -> ActivityGroupCard and hide the synthetic retry message in the visible feed. Make model/provider selection platform-aware: add desktop platform helpers, hide UI-disabled providers and exclude Antigravity on Windows. Remove Antigravity catalog entry for unsupported platforms. Bump Codex app-server managed version to 0.142.3 (update checksums and remove managed args). Update and add tests to cover new retry behavior, UI markup, and platform-specific provider filtering.
