feat(ui): month card grid + week timeline, wire view switcher

Replace the throwaway debug screen with the first real calendar UI and a
functional Month <-> Week switcher, on Material 3 Expressive.

Month view (S1):
- Material 3 Expressive card-per-day grid; only the current month's weeks
  render (neighbouring days left blank)
- per-day event dots with "+N" overflow, today via primaryContainer
- spring-based press feedback from the active motion scheme
- swipe + drawer navigation, Loading/Failure/Success states

Week view (S2):
- vertical time schedule with overlap-resolved lanes (per-day clipping,
  midnight spanning, instant events)
- all-day / multi-day events as connected horizontal spans
- single scroll container (gutter + day columns stay aligned), columns
  bundled in a rounded container, noon-centred on load
- top section colour-shifts with the app bar on scroll; swipe navigation,
  three states

Shared / infra:
- CalendarHost holds the active view; RootScreen renders it post-permission
- ui/common building blocks: CalendarDrawer, CalendarFailure,
  ViewSwitcherPill, pastelize, observable locale, M3 Expressive slide
  transition (motionScheme fastSpatialSpec)
- unit tests for the week layout (lanes, clipping, all-day spans)
- build: compileSdk 37, material3 pinned to 1.5.0-alpha21 for Expressive

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -0,0 +1,36 @@
+package de.jeanlucmakiola.calendula.ui
+
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.saveable.rememberSaveable
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Modifier
+import de.jeanlucmakiola.calendula.ui.common.CalendarView
+import de.jeanlucmakiola.calendula.ui.month.MonthScreen
+import de.jeanlucmakiola.calendula.ui.week.WeekScreen
+
+/**
+ * Holds the active top-level view (spec M1) and swaps between the calendar
+ * screens. Each screen owns its own ViewModel and date anchor; the view-switcher
+ * pill in their top bars writes back here via [onSelectView].
+ */
+@Composable
+fun CalendarHost(modifier: Modifier = Modifier) {
+    var view by rememberSaveable { mutableStateOf(CalendarView.Month) }
+    val onSelectView: (CalendarView) -> Unit = { view = it }
+
+    when (view) {
+        CalendarView.Week -> WeekScreen(
+            selectedView = view,
+            onSelectView = onSelectView,
+            modifier = modifier,
+        )
+        // Month, plus Day as a fallback until the day view lands (v0.5).
+        else -> MonthScreen(
+            selectedView = view,
+            onSelectView = onSelectView,
+            modifier = modifier,
+        )
+    }
+}