fix: center splash logo and wordmark (stack was sized to children)

Wrap AnimatedBuilder in SizedBox.expand so the Stack fills the full
Scaffold body; alignment: Alignment.center now centers within the whole
screen instead of within the wordmark-sized intrinsic box.

Adds a regression widget test (test/ui/splash_screen_test.dart) that
asserts the wordmark dx is within 1px of screen-center at 1500ms into
the animation.

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

lib/ui/screens/splash_screen.dart

@@ -57,7 +57,8 @@ class _SplashScreenState extends State<SplashScreen>
     const gap = 3.0;
     return Scaffold(
       backgroundColor: const Color(0xFF0E1430),
-      body: AnimatedBuilder(
+      body: SizedBox.expand(
+        child: AnimatedBuilder(
           animation: _c,
           builder: (context, _) {
             final titleT = const Interval(0.60, 0.88, curve: Curves.easeOut)
@@ -86,6 +87,7 @@ class _SplashScreenState extends State<SplashScreen>
             );
           },
         ),
+      ),
     );
   }
 

test/ui/splash_screen_test.dart

@@ -0,0 +1,19 @@
+import 'package:block_seasons/ui/screens/splash_screen.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('splash wordmark is horizontally centered', (tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: SplashScreen(nextScreen: () => const Scaffold(body: SizedBox())),
+    ));
+    // Mid-animation: wordmark already laid out.
+    await tester.pump(const Duration(milliseconds: 1500));
+    final screenWidth = tester.getSize(find.byType(SplashScreen)).width;
+    final wordmark = tester.getCenter(find.text('BLOCK SEASONS'));
+    expect(wordmark.dx, closeTo(screenWidth / 2, 1.0));
+    // Drain the rest of the animation so no timers leak.
+    await tester.pump(const Duration(milliseconds: 600));
+    await tester.pumpAndSettle();
+  });
+}