Use ContentView and add station input UI

- Replace the placeholder window content with ContentView and implement a basic station lookup UI.
- Added a station code TextField, Refresh button that triggers an async load, loading indicator, and error message display. Introduces a Table for TrainStatusRow with columns for number, route, on-time and platform.
- Started plumbing for row population (work in progress).
- Also tweaked window title and fixes station comment (Ashland).

Sources/AmtrakStatus/AmtrakStatusApp.swift

@@ -4,9 +4,8 @@ import DefaultBackend
 @main
 struct AmtrakStatusApp: App {
     var body: some Scene {
-        WindowGroup("AmtrakStatus") {
-            Text("Hello, World!")
-                .padding()
+        WindowGroup("Amtrak Status") {
+            ContentView()
         }
     }
 }