feat: serpentine journey map with auto-scroll and glowing current node

Replaces the plain GridView with a Candy-Crush-style journey map:
dotted serpentine path, circular nodes (gold=done, glowing=current,
dark+lock=locked), glass header, auto-scroll to current stage.
Updates season_map_screen_test to use Key('stage_node_$i') finders.

test/ui/season_map_screen_test.dart

@@ -71,13 +71,28 @@ void main() {
     );
     await tester.pumpAndSettle();
 
+    // Total stars displayed in header.
     expect(find.text('★ 2/9'), findsOneWidget);
-    expect(find.byIcon(Icons.lock), findsOneWidget); // stage 3 locked
-    expect(find.text('1'), findsOneWidget);
-    expect(find.text('2'), findsOneWidget);
 
-    // Stage 1 is starred, so stage 2 is unlocked and playable.
-    await tester.tap(find.text('2'));
+    // Node 0 (stage 1) exists.
+    expect(find.byKey(const Key('stage_node_0')), findsOneWidget);
+
+    // Stage 3 (index 2) is locked — contains a lock icon.
+    expect(
+      find.descendant(
+        of: find.byKey(const Key('stage_node_2')),
+        matching: find.byIcon(Icons.lock),
+      ),
+      findsOneWidget,
+    );
+
+    // Stage 1 is starred, so stage 2 (index 1) is unlocked and playable.
+    // Ensure the node is visible before tapping.
+    await tester.ensureVisible(find.byKey(const Key('stage_node_1')));
+    await tester.tap(
+      find.byKey(const Key('stage_node_1')),
+      warnIfMissed: false,
+    );
     await tester.pumpAndSettle();
 
     expect(find.byType(GameScreen), findsOneWidget);