fix: 标题文本显式指定 onBackground 颜色，适配深色主题

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

shared/src/commonMain/kotlin/plus/rua/project/ui/MonthHeader.kt

@@ -65,6 +65,7 @@ fun MonthHeader(
         ) { (y, m) ->
             Text(
                 text = "${y}年${m}月",
+                color = MaterialTheme.colorScheme.onBackground,
                 style = MaterialTheme.typography.titleLarge
             )
         }
@@ -84,6 +85,7 @@ fun MonthHeader(
         ) { week ->
             Text(
                 text = "第${week}周",
+                color = MaterialTheme.colorScheme.onBackground,
                 style = MaterialTheme.typography.bodySmall
             )
         }

shared/src/commonMain/kotlin/plus/rua/project/ui/YearGridView.kt

@@ -347,6 +347,7 @@ fun YearHeader(
             ) { y ->
                 Text(
                     text = "${y}年",
+                    color = MaterialTheme.colorScheme.onBackground,
                     style = MaterialTheme.typography.titleLarge,
                     fontWeight = FontWeight.Bold
                 )