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>

gradle/libs.versions.toml

@@ -41,6 +41,7 @@ androidx-ui-test-manifest = { group = "androidx.compose.ui", name = "ui-test-man
 
 # Material 3 (Expressive lives in this artifact for 1.5+)
 androidx-material3 = { group = "androidx.compose.material3", name = "material3", version.ref = "material3" }
+androidx-compose-material-icons-core = { group = "androidx.compose.material", name = "material-icons-core" }
 
 # Hilt
 hilt-android = { group = "com.google.dagger", name = "hilt-android", version.ref = "hilt" }