TUI Plugin Sharing 2 - add remote plugin section plumbing (#26702)

This adds the background plumbing for remote-backed plugin catalog
sections while leaving the fuller directory presentation to the next PR.
The TUI can fetch section-specific remote marketplace results, keep
local plugin data available, and carry section errors forward for later
rendering.

- Fetches explicit remote marketplace kinds for curated, workspace, and
shared-with-me sections.
- Gates shared-with-me loading on the plugin sharing feature flag.
- Adds section-level error state and user-actionable error copy.
- Merges remote marketplace results into the cached plugin list without
discarding local results.

codex-rs/tui/src/chatwidget/constructor.rs

@@ -160,6 +160,9 @@ impl ChatWidget {
             ide_context: IdeContextState::default(),
             plugins_cache: PluginsCacheState::default(),
             plugins_fetch_state: PluginListFetchState::default(),
+            plugin_remote_sections_loading: false,
+            plugin_remote_sections_loaded: false,
+            plugin_remote_section_errors: Vec::new(),
             plugin_install_apps_needing_auth: Vec::new(),
             plugin_install_auth_flow: None,
             plugins_active_tab_id: None,