feat(nav): default view setting + widget-aware back stack

Replace CalendarHost's single global view slot with a top-level view
back stack rooted at a user-configurable default view. A lateral move
(pill/drawer/widget) replaces the non-home top; a date tap drills the
day view on top; a base-level BackHandler pops one level until only the
home view remains, then the system exits.

Widgets now carry their source view (EXTRA_SOURCE_VIEW) so a launch
roots the stack in that widget's view: backing out of a day/event opened
from the agenda widget returns to Agenda, and from the month widget to
Month, instead of always landing on Week. Reminder taps keep the
separate detail-key channel and leave the base view untouched.

Add a Default view setting (Settings -> Appearance) backing the stack's
home view; defaults to Week so existing users see no change.

Resolves the two Codeberg reports from @devinside:
- #1 [FR] Option to set default view
- #2 [Bug] Widget UX improvement

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

app/src/test/java/de/jeanlucmakiola/calendula/ui/common/ViewBackStackTest.kt

@@ -0,0 +1,75 @@
+package de.jeanlucmakiola.calendula.ui.common
+
+import com.google.common.truth.Truth.assertThat
+import org.junit.jupiter.api.Test
+
+/**
+ * The pure back-stack rules behind [CalendarHost]'s top-level navigation:
+ * the home view is always the bottom, a lateral move keeps the stack one deep,
+ * and a date tap drills the day view on top. Verifies the back sequences the
+ * widget-UX fix promises (e.g. agenda widget → day → back → agenda → back → home).
+ */
+class ViewBackStackTest {
+
+    @Test
+    fun `lateral move to the home view collapses to just the home`() {
+        assertThat(viewBaseStack(CalendarView.Week, CalendarView.Week))
+            .containsExactly(CalendarView.Week)
+    }
+
+    @Test
+    fun `lateral move to a non-home view layers it over the home`() {
+        assertThat(viewBaseStack(CalendarView.Week, CalendarView.Month))
+            .containsExactly(CalendarView.Week, CalendarView.Month)
+            .inOrder()
+    }
+
+    @Test
+    fun `a lateral move never grows past one level above home`() {
+        // Switching pill targets in turn replaces the top rather than stacking.
+        val afterMonth = viewBaseStack(CalendarView.Week, CalendarView.Month)
+        val afterAgenda = viewBaseStack(CalendarView.Week, CalendarView.Agenda)
+        assertThat(afterMonth).hasSize(2)
+        assertThat(afterAgenda).containsExactly(CalendarView.Week, CalendarView.Agenda).inOrder()
+    }
+
+    @Test
+    fun `drilling pushes the day view on top`() {
+        assertThat(listOf(CalendarView.Week, CalendarView.Month).drillToDay())
+            .containsExactly(CalendarView.Week, CalendarView.Month, CalendarView.Day)
+            .inOrder()
+    }
+
+    @Test
+    fun `drilling is a no-op when the day view is already current`() {
+        val stack = listOf(CalendarView.Week, CalendarView.Day)
+        assertThat(stack.drillToDay()).isEqualTo(stack)
+    }
+
+    @Test
+    fun `agenda widget date tap backs out agenda then home`() {
+        // openDate(source = Agenda): root in Agenda over the Week home, then drill.
+        val stack = viewBaseStack(CalendarView.Week, CalendarView.Agenda).drillToDay()
+        assertThat(stack)
+            .containsExactly(CalendarView.Week, CalendarView.Agenda, CalendarView.Day)
+            .inOrder()
+        // Pressing back pops one level at a time down to the home view.
+        assertThat(stack.dropLast(1)).containsExactly(CalendarView.Week, CalendarView.Agenda).inOrder()
+        assertThat(stack.dropLast(2)).containsExactly(CalendarView.Week)
+    }
+
+    @Test
+    fun `month widget date tap backs out month then home`() {
+        val stack = viewBaseStack(CalendarView.Week, CalendarView.Month).drillToDay()
+        assertThat(stack.last()).isEqualTo(CalendarView.Day)
+        assertThat(stack.dropLast(1).last()).isEqualTo(CalendarView.Month)
+        assertThat(stack.first()).isEqualTo(CalendarView.Week)
+    }
+
+    @Test
+    fun `a widget whose view is the home view does not duplicate the home`() {
+        // Default = Agenda, agenda widget event tap → just the home, no extra layer.
+        assertThat(viewBaseStack(CalendarView.Agenda, CalendarView.Agenda))
+            .containsExactly(CalendarView.Agenda)
+    }
+}