feat: make text in the GUI app selectable (Closes #15)

Wrap the main content Column in SelectionContainer to enable text selection and copying for all Text composables in the Compose Desktop GUI.

gui/src/main/kotlin/de/pfadfinder/songbook/gui/App.kt

@@ -2,6 +2,7 @@ package de.pfadfinder.songbook.gui
 
 import androidx.compose.desktop.ui.tooling.preview.Preview
 import androidx.compose.foundation.VerticalScrollbar
+import androidx.compose.foundation.text.selection.SelectionContainer
 import androidx.compose.foundation.layout.*
 import androidx.compose.foundation.lazy.LazyColumn
 import androidx.compose.foundation.lazy.items
@@ -85,6 +86,7 @@ fun App() {
 
     MaterialTheme {
         Surface(modifier = Modifier.fillMaxSize()) {
+            SelectionContainer {
             Column(modifier = Modifier.padding(16.dp)) {
                 // Project directory selection
                 Text(
@@ -336,6 +338,7 @@ fun App() {
                     )
                 }
             }
+            }
         }
     }
 }