style(month): drop the split style's divider (#53)

The rule between the compact grid and the day pane earns its place in the
Continuous header, where it closes a section off; here it only draws a line
between two halves the layout already separates.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/java/de/jeanlucmakiola/calendula/ui/month/MonthScreen.kt

@@ -836,7 +836,6 @@ private fun SplitMonthContent(
                 // full of tappable rows, so it shouldn't also be a month gesture.
                 modifier = rememberMonthSwipeModifier(onSwipeNext, onSwipePrev),
             )
-            HorizontalDivider(color = MaterialTheme.colorScheme.outlineVariant)
             SplitDayPane(
                 date = selected,
                 today = state.today,

app/src/main/java/de/jeanlucmakiola/calendula/ui/month/MonthStylePreview.kt

@@ -7,7 +7,6 @@ import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.height
 import androidx.compose.foundation.lazy.rememberLazyListState
-import androidx.compose.material3.HorizontalDivider
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.remember
@@ -138,7 +137,6 @@ internal fun MonthStylePreview(
                             showWeekNumbers = false,
                             onSelectDay = {},
                         )
-                        HorizontalDivider(color = MaterialTheme.colorScheme.outlineVariant)
                         SplitDayPane(
                             date = today,
                             today = today,