This PR adds marketplace upgrade to the /plugins menu so users can update configured marketplaces. It adds a Ctrl+U shortcut on eligible marketplace tabs, a loading state, and the app-server request flow needed to perform marketplace/upgrade. After a successful upgrade, the TUI refreshes plugin data, plugin mentions, and user config so updated marketplace contents show up across the menu and other plugin surfaces. It also preserves the current marketplace tab on no-op and failure paths and surfaces backend error details directly in the TUI.

• Add a Ctrl+U upgrade option for user-configured marketplace tabs in /plugins
• Show the upgrade footer hint only on upgradeable marketplace tabs
• Show a loading state during marketplace/upgrade
• Surface already-up-to-date and per-marketplace failure results from the backend
• Refresh plugin data, plugin mentions, and user config after successful upgrades
• Add tests and snapshot updates for the shortcut flow, loading state, and failure messaging

Steps to test:

1. Add a /plugin marketplace to Codex TUI.
2. Open /plugins, move to that marketplace tab, and confirm the footer shows Ctrl+U to upgrade.
3. Press Ctrl+U and confirm the popup switches into an upgrade loading state.
4. When the request finishes, confirm you see the expected result: updated marketplace contents on success, an already-up-to-date message on no-op, or backend error details on failure. On no-op or failure, confirm the popup stays on the same marketplace tab.