feat: compute birthday flag in LunarCache

core/src/main/kotlin/plus/rua/project/LunarCache.kt

@@ -75,23 +75,25 @@ class LunarCache(
         val lunarDay = solarDay.getLunarDay()
         val lunarMonth = lunarDay.getLunarMonth()
         val lunarMonthName = lunarMonth.getName()
+        val isBirthday = (date.month.number == 9 && date.day == 4) ||
+            (lunarDay.getLunarMonth().getIndexInYear() == 0 && lunarDay.day == 21)
 
         // 农历传统节日（仅当天）
         val lunarFestival = lunarDay.getFestival()
         if (lunarFestival != null) {
-            return DayCellInfo(lunarFestival.getName(), true, holidayBadge, lunarMonthName)
+            return DayCellInfo(lunarFestival.getName(), true, holidayBadge, lunarMonthName, isBirthday)
         }
 
         // 节气（当天才显示）
         val termDay = solarDay.getTermDay()
         if (termDay.getDayIndex() == 0) {
-            return DayCellInfo(termDay.getSolarTerm().getName(), true, holidayBadge, lunarMonthName)
+            return DayCellInfo(termDay.getSolarTerm().getName(), true, holidayBadge, lunarMonthName, isBirthday)
         }
 
         // 公历节日（仅当天）
         val solarFestival = solarDay.getFestival()
         if (solarFestival != null) {
-            return DayCellInfo(solarFestival.getName(), true, holidayBadge, lunarMonthName)
+            return DayCellInfo(solarFestival.getName(), true, holidayBadge, lunarMonthName, isBirthday)
         }
 
         // 默认：农历日期
@@ -101,7 +103,7 @@ class LunarCache(
         } else {
             name
         }
-        return DayCellInfo(text, false, holidayBadge, lunarMonthName)
+        return DayCellInfo(text, false, holidayBadge, lunarMonthName, isBirthday)
     }
 
     companion object {
@@ -121,5 +123,6 @@ data class DayCellInfo(
     val annotationText: String,
     val isAnnotationHighlight: Boolean,
     val holidayBadge: String?,
-    val lunarMonthName: String? = null
+    val lunarMonthName: String? = null,
+    val isBirthday: Boolean = false
 )

core/src/test/kotlin/plus/rua/project/LunarCacheBirthdayTest.kt

@@ -0,0 +1,42 @@
+package plus.rua.project
+
+import kotlinx.coroutines.test.runTest
+import kotlinx.datetime.LocalDate
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class LunarCacheBirthdayTest {
+    private val cache = LunarCache()
+
+    @Test
+    fun solarBirthdaySeptember4_returnsTrue() = runTest {
+        val info = cache.getOrCompute(LocalDate(2026, 9, 4))
+        assertTrue("阳历 9 月 4 日应为生日", info.isBirthday)
+    }
+
+    @Test
+    fun lunarBirthdayFirstMonthDay21_returnsTrue() = runTest {
+        // 2026 年农历正月二十一对应阳历 2026-03-09
+        val info = cache.getOrCompute(LocalDate(2026, 3, 9))
+        assertTrue("农历正月二十一应为生日", info.isBirthday)
+    }
+
+    @Test
+    fun regularDate_returnsFalse() = runTest {
+        val info = cache.getOrCompute(LocalDate(2026, 6, 15))
+        assertFalse("普通日期不应为生日", info.isBirthday)
+    }
+
+    @Test
+    fun solarBirthdayNotFirstLunarDay21_stillReturnsTrue() = runTest {
+        val info = cache.getOrCompute(LocalDate(2026, 9, 4))
+        assertTrue(info.isBirthday)
+    }
+
+    @Test
+    fun lunarBirthdayNotSeptember4_stillReturnsTrue() = runTest {
+        val info = cache.getOrCompute(LocalDate(2026, 3, 9))
+        assertTrue(info.isBirthday)
+    }
+}