feat(prescription): expand canvas to fill bottom space and add cursor-aware newline on bullet insertion

test/features/prescription_test.dart

@@ -79,7 +79,7 @@ void main() {
       expect(find.text('ESC para salir'), findsOneWidget);
     });
 
-    testWidgets('inserts bullet when clicking viñeta button', (tester) async {
+    testWidgets('inserts bullet when clicking viñeta button and jumps to new line if text has content', (tester) async {
       final configStore = ConfigStore();
       await tester.pumpWidget(
         MaterialApp(
@@ -100,6 +100,18 @@ void main() {
       await tester.pump();
 
       expect(find.text('• '), findsOneWidget);
+
+      // Now enter text at end of line and test auto-newline
+      final prescriptionField = find.byWidgetPredicate(
+        (w) => w is TextField && (w.decoration?.hintText?.contains('Ciprofloxacina') ?? false),
+      );
+      await tester.enterText(prescriptionField, '1. Ciprofloxacina 500mg');
+      await tester.pump();
+
+      await tester.tap(bulletBtn);
+      await tester.pump();
+
+      expect(find.text('1. Ciprofloxacina 500mg\n• '), findsOneWidget);
     });
 
     testWidgets('inserts sequential numbers and sub-indication dosis', (tester) async {