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. Song titles, filenames, status messages, and section labels are
now selectable, while buttons and text fields continue to work normally.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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(
@@ -339,6 +341,7 @@ fun App() {
             }
         }
     }
+}
 
 enum class MessageType {
     INFO, SUCCESS, ERROR