refactor(tui): extract model selector UI into dedicated widget module

Added `widgets::model_picker` containing the full model picker rendering logic and moved related helper functions there. Updated `ui.rs` to use `render_model_picker` and removed the now‑duplicate model selector implementation. This cleanly separates UI concerns and improves code reuse.

crates/owlen-tui/src/commands/mod.rs

@@ -148,6 +148,10 @@ const COMMANDS: &[CommandSpec] = &[
         keyword: "models --cloud",
         description: "Open model picker focused on cloud models",
     },
+    CommandSpec {
+        keyword: "models --available",
+        description: "Open model picker showing available models",
+    },
     CommandSpec {
         keyword: "new",
         description: "Start a new conversation",