feat(nav): interactive month widget + history-retracing back

On-device review follow-ups:

- Month widget grid is now tappable. Day numbers open that day and event
  bars open the event's detail, both rooted in the month view so back
  returns to the grid. Previously only the prev/next/today header
  controls responded — the grid cells were never clickable.

- Pill/drawer view switches now build a visit history instead of
  collapsing to the default view. Back retraces the views you moved
  through (a not-yet-visited view is pushed; revisiting one collapses the
  loop back to it), down to the default, then exits. Widget launches
  still reset to their own view context.

Refs #1, #2 (reported by @devinside).

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

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

@@ -12,25 +12,44 @@ import org.junit.jupiter.api.Test
 class ViewBackStackTest {
 
     @Test
-    fun `lateral move to the home view collapses to just the home`() {
+    fun `a widget whose view is the home view resets 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`() {
+    fun `a widget launch resets to its view 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()
+    fun `pill switches build a visit history that back retraces`() {
+        // Agenda → Day → Week → Month, each a new view, stacks the full history.
+        val stack = listOf(CalendarView.Agenda)
+            .selectView(CalendarView.Day)
+            .selectView(CalendarView.Week)
+            .selectView(CalendarView.Month)
+        assertThat(stack).containsExactly(
+            CalendarView.Agenda, CalendarView.Day, CalendarView.Week, CalendarView.Month,
+        ).inOrder()
+        // Back walks the views in reverse, not straight to the home view.
+        assertThat(stack.dropLast(1).last()).isEqualTo(CalendarView.Week)
+    }
+
+    @Test
+    fun `selecting an already-visited view collapses the loop back to it`() {
+        val stack = listOf(CalendarView.Week, CalendarView.Agenda, CalendarView.Day)
+            .selectView(CalendarView.Agenda)
+        assertThat(stack).containsExactly(CalendarView.Week, CalendarView.Agenda).inOrder()
+    }
+
+    @Test
+    fun `selecting the home view collapses to just the home`() {
+        val stack = listOf(CalendarView.Week, CalendarView.Agenda, CalendarView.Day)
+            .selectView(CalendarView.Week)
+        assertThat(stack).containsExactly(CalendarView.Week)
     }
 
     @Test