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

@@ -57,34 +57,36 @@ class _SplashScreenState extends State<SplashScreen>
     const gap = 3.0;
     return Scaffold(
       backgroundColor: const Color(0xFF0E1430),
-      body: AnimatedBuilder(
-        animation: _c,
-        builder: (context, _) {
-          final titleT = const Interval(0.60, 0.88, curve: Curves.easeOut)
-              .transform(_c.value);
-          return Stack(
-            alignment: Alignment.center,
-            children: [
-              for (var i = 0; i < _blocks.length; i++)
-                _block(i, blockSize, gap),
-              Transform.translate(
-                offset: Offset(0, 78 + 12 * (1 - titleT)),
-                child: Opacity(
-                  opacity: titleT,
-                  child: const Text(
-                    'BLOCK SEASONS',
-                    style: TextStyle(
-                      fontSize: 26,
-                      fontWeight: FontWeight.w900,
-                      letterSpacing: 4,
-                      color: Colors.white,
+      body: SizedBox.expand(
+        child: AnimatedBuilder(
+          animation: _c,
+          builder: (context, _) {
+            final titleT = const Interval(0.60, 0.88, curve: Curves.easeOut)
+                .transform(_c.value);
+            return Stack(
+              alignment: Alignment.center,
+              children: [
+                for (var i = 0; i < _blocks.length; i++)
+                  _block(i, blockSize, gap),
+                Transform.translate(
+                  offset: Offset(0, 78 + 12 * (1 - titleT)),
+                  child: Opacity(
+                    opacity: titleT,
+                    child: const Text(
+                      'BLOCK SEASONS',
+                      style: TextStyle(
+                        fontSize: 26,
+                        fontWeight: FontWeight.w900,
+                        letterSpacing: 4,
+                        color: Colors.white,
+                      ),
                     ),
                   ),
                 ),
-              ),
-            ],
-          );
-        },
+              ],
+            );
+          },
+        ),
       ),
     );
   }

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();
+  });
+}