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/main/java/de/jeanlucmakiola/calendula/ui/CalendarHost.kt

@@ -26,6 +26,7 @@ import de.jeanlucmakiola.calendula.ui.calendars.CalendarsScreen
 import de.jeanlucmakiola.calendula.ui.common.CalendarView
 import de.jeanlucmakiola.calendula.ui.common.drillToDay
 import de.jeanlucmakiola.calendula.ui.common.rememberCalendarSlideSpec
+import de.jeanlucmakiola.calendula.ui.common.selectView
 import de.jeanlucmakiola.calendula.ui.common.viewBaseStack
 import de.jeanlucmakiola.calendula.ui.day.DayScreen
 import de.jeanlucmakiola.calendula.ui.detail.EventDetailScreen
@@ -46,11 +47,12 @@ import kotlin.time.Clock
  * pill in their top bars writes back here via [onSelectView].
  *
  * The stack's bottom is the user's [CalendarHostViewModel.defaultView] home view.
- * A lateral move (pill / drawer / a widget establishing its own view) replaces
- * the non-home top; a date tap drills the day view on top. Pressing back pops
- * one level, and the base-level [BackHandler] hands off to the system to exit
- * once only the home view remains. So back from a widget-opened screen returns
- * to that widget's view, then home, then out.
+ * A lateral switch (pill / drawer) builds a visit history so back retraces it
+ * (see [selectView]); a widget launch resets the stack to its own view; a date
+ * tap drills the day view on top. Pressing back pops one level, and the
+ * base-level [BackHandler] hands off to the system to exit once only the home
+ * view remains. So back from a widget-opened screen returns to that widget's
+ * view, then home; and back through pill switches walks the views in reverse.
  *
  * [requestedDetailKey] is an externally requested occurrence (a tapped
  * reminder notification routed through MainActivity): it opens the detail
@@ -79,7 +81,7 @@ fun CalendarHost(
         mutableStateOf(listOf(defaultView))
     }
     val view = viewStack.last()
-    val onSelectView: (CalendarView) -> Unit = { viewStack = viewBaseStack(defaultView, it) }
+    val onSelectView: (CalendarView) -> Unit = { viewStack = viewStack.selectView(it) }
 
     // Tapping a day in the month grid opens the day view anchored to that date.
     var pendingDayIso by rememberSaveable { mutableStateOf<String?>(null) }