feat(settings): allow week to start on any day

Replace the AUTO/MONDAY/SUNDAY week-start enum with a sealed type
(Auto + Day(DayOfWeek)) so users can pick any of the seven days as the
first day of the week. The picker now lists "Automatic" plus all seven
localised weekday names; labels come from java.time display names rather
than per-day string resources.

Stored values round-trip by DayOfWeek.name, so the legacy MONDAY/SUNDAY
preferences migrate transparently. Garbage values fall back to Auto.

Closes #3 (Codeberg)

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

app/src/main/java/de/jeanlucmakiola/calendula/data/prefs/SettingsPrefs.kt

@@ -19,18 +19,23 @@ import javax.inject.Singleton
 /** Light/dark override. SYSTEM follows the device setting. */
 enum class ThemeMode { SYSTEM, LIGHT, DARK }
 
-/** Week-start override. AUTO derives the first day from the active locale. */
-enum class WeekStartPref { AUTO, MONDAY, SUNDAY }
+/**
+ * Week-start override. [Auto] derives the first day from the active locale;
+ * [Day] pins a specific weekday (any of the seven).
+ */
+sealed interface WeekStartPref {
+    data object Auto : WeekStartPref
+    data class Day(val day: DayOfWeek) : WeekStartPref
+}
 
 /**
- * Resolve the preference to a concrete first-day-of-week. AUTO reads the
- * locale's convention (e.g. Monday in DE, Sunday in en-US).
+ * Resolve the preference to a concrete first-day-of-week. [WeekStartPref.Auto]
+ * reads the locale's convention (e.g. Monday in DE, Sunday in en-US).
  */
 fun WeekStartPref.resolveFirstDay(locale: Locale): DayOfWeek = when (this) {
-    WeekStartPref.MONDAY -> DayOfWeek.MONDAY
-    WeekStartPref.SUNDAY -> DayOfWeek.SUNDAY
+    is WeekStartPref.Day -> day
     // java.time.DayOfWeek.value is ISO 1..7 (Mon..Sun) — same numbering kotlinx uses.
-    WeekStartPref.AUTO -> DayOfWeek(WeekFields.of(locale).firstDayOfWeek.value)
+    WeekStartPref.Auto -> DayOfWeek(WeekFields.of(locale).firstDayOfWeek.value)
 }
 
 /**
@@ -55,7 +60,7 @@ class SettingsPrefs @Inject constructor(
     }
 
     val weekStart: Flow<WeekStartPref> = store.data.map { prefs ->
-        prefs[WEEK_START_KEY].toEnum(WeekStartPref.AUTO)
+        parseWeekStart(prefs[WEEK_START_KEY])
     }
 
     suspend fun setThemeMode(mode: ThemeMode) {
@@ -67,7 +72,7 @@ class SettingsPrefs @Inject constructor(
     }
 
     suspend fun setWeekStart(pref: WeekStartPref) {
-        store.edit { it[WEEK_START_KEY] = pref.name }
+        store.edit { it[WEEK_START_KEY] = pref.storageValue() }
     }
 
     /**
@@ -331,6 +336,25 @@ private fun MutableMap<Long, Int?>.applyOverride(
     }
 }
 
+/** Sentinel stored for [WeekStartPref.Auto]; days store their [DayOfWeek.name]. */
+private const val WEEK_START_AUTO = "AUTO"
+
+private fun WeekStartPref.storageValue(): String = when (this) {
+    WeekStartPref.Auto -> WEEK_START_AUTO
+    is WeekStartPref.Day -> day.name
+}
+
+/**
+ * Parse the stored week-start value. "AUTO"/null/garbage → [WeekStartPref.Auto];
+ * a [DayOfWeek] name → [WeekStartPref.Day]. The legacy "MONDAY"/"SUNDAY" enum
+ * values migrate transparently, since both are valid day names.
+ */
+private fun parseWeekStart(stored: String?): WeekStartPref = when (stored) {
+    null, WEEK_START_AUTO -> WeekStartPref.Auto
+    else -> DayOfWeek.entries.firstOrNull { it.name == stored }
+        ?.let { WeekStartPref.Day(it) } ?: WeekStartPref.Auto
+}
+
 private const val NONE = "none"
 private const val ENTRY_SEP = ";"
 private const val KEY_VALUE_SEP = "="

app/src/main/java/de/jeanlucmakiola/calendula/ui/settings/SettingsScreen.kt

@@ -105,7 +105,10 @@ import de.jeanlucmakiola.calendula.ui.common.TimePickerAlert
 import de.jeanlucmakiola.calendula.ui.common.positionOf
 import de.jeanlucmakiola.calendula.ui.common.reminderLeadTimeLabel
 import de.jeanlucmakiola.calendula.ui.common.rememberCalendarSlideSpec
+import de.jeanlucmakiola.calendula.ui.common.currentLocale
+import kotlinx.datetime.DayOfWeek
 import kotlinx.datetime.LocalTime
+import java.time.format.TextStyle as JavaTextStyle
 import java.util.Calendar
 
 /** The settings sub-screens reached from the hub's category rows. */
@@ -484,7 +487,7 @@ private fun AppearanceScreen(
     if (showWeekStart) {
         OptionPicker(
             title = stringResource(R.string.settings_week_start),
-            options = WeekStartPref.entries,
+            options = WEEK_START_OPTIONS,
             selected = state.weekStart,
             label = { weekStartLabel(it) },
             onSelect = viewModel::setWeekStart,
@@ -993,14 +996,18 @@ private fun themeLabel(mode: ThemeMode): String = stringResource(
     },
 )
 
+/** Picker options: "Follow system" first, then Monday…Sunday in ISO order. */
+private val WEEK_START_OPTIONS: List<WeekStartPref> =
+    listOf(WeekStartPref.Auto) + DayOfWeek.entries.map { WeekStartPref.Day(it) }
+
 @Composable
-private fun weekStartLabel(pref: WeekStartPref): String = stringResource(
-    when (pref) {
-        WeekStartPref.AUTO -> R.string.settings_week_start_auto
-        WeekStartPref.MONDAY -> R.string.settings_week_start_monday
-        WeekStartPref.SUNDAY -> R.string.settings_week_start_sunday
-    },
-)
+private fun weekStartLabel(pref: WeekStartPref): String = when (pref) {
+    WeekStartPref.Auto -> stringResource(R.string.settings_week_start_auto)
+    // Localised full weekday name, so any of the seven days reads naturally
+    // without a per-day string resource.
+    is WeekStartPref.Day -> java.time.DayOfWeek.of(pref.day.ordinal + 1)
+        .getDisplayName(JavaTextStyle.FULL, currentLocale())
+}
 
 @Composable
 private fun languageLabel(tag: String?): String =

app/src/main/java/de/jeanlucmakiola/calendula/ui/settings/SettingsUiState.kt

@@ -17,7 +17,7 @@ data class SettingsUiState(
     val themeMode: ThemeMode = ThemeMode.SYSTEM,
     val dynamicColor: Boolean = true,
     val dynamicColorAvailable: Boolean = true,
-    val weekStart: WeekStartPref = WeekStartPref.AUTO,
+    val weekStart: WeekStartPref = WeekStartPref.Auto,
     /** The calendar view the app opens on, and the home of the view back stack (M1). */
     val defaultView: CalendarView = CalendarView.Week,
     /** Optional event-form fields shown by default (rest behind "more fields"). */