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>

CHANGELOG.md

@@ -7,6 +7,22 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Month view (S1): Material 3 Expressive card-per-day grid (only the current
+  month's weeks; neighbouring days left blank), per-day event dots with "+N"
+  overflow, today emphasised 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,
+  separate all-day strip, midnight-spanning events clipped per day, swipe
+  navigation, Loading/Failure/Success states
+- Functional view-switcher (M1) cycling Month ↔ Week (Day joins with S3)
+- Shared calendar UI building blocks in `ui/common/` (navigation drawer,
+  failure screen, view-switcher pill, color pastelizer, observable locale)
+
+### Removed
+- Throwaway debug screen — superseded by the month view
+
 ## [0.2.1] — 2026-06-09
 
 ### Changed