Extract TUI plugin catalog rendering (#28768)

This mechanically extracts the existing TUI plugin catalog and detail
popup rendering from `chatwidget/plugins.rs` into a new
`chatwidget/plugin_catalog.rs` module. `plugins.rs` now keeps the
stateful plugin workflow and orchestration, while `plugin_catalog.rs`
owns the presentation-heavy catalog/detail popup construction and its
pure helpers. The goal is to keep `plugins.rs` focused before later
plugin sharing work adds more catalog behavior.

- Moves existing catalog/detail popup builders and related pure helpers
into `plugin_catalog.rs`
- Leaves plugin fetch/state/key handling in `plugins.rs`
- Adds only minimal sibling-module visibility/import wiring
- Intentionally makes no product behavior or UI changes beyond the code
move

codex-rs/tui/src/chatwidget.rs

@@ -365,6 +365,7 @@ use self::skills::collect_tool_mentions;
 use self::skills::find_app_mentions;
 use self::skills::find_skill_mentions_with_tool_mentions;
 use self::skills::is_app_mentionable;
+mod plugin_catalog;
 mod plugins;
 use self::plugins::PluginInstallAuthFlowState;
 use self::plugins::PluginListFetchState;