feat(android): bring the Android app level with the iOS app

Everything the iOS shell gained, built natively for Android 16 with Material
You, and back to Android 7:

- Home-screen widgets in Jetpack Glance: trip, today, bookings, to-dos,
  packing, world map and travel group, from the same snapshot the iOS widgets
  read. Wallpaper colours on Android 12+, resizable in three layouts, a trip
  picker per widget (reconfigurable), live previews in Android 15's picker
  and preview images before it.
- The travel day as a Live Update: a promoted ongoing notification with the
  next stop as the status-bar chip, a countdown and the day's stops as
  segments (ProgressStyle), a quiet ongoing notification before Android 16.
- A share sheet of its own: a Material bottom sheet with the trip picker for
  files, mails, pages and links shared from any app, and the coming trips as
  launcher and direct-share shortcuts.
- Push: channels per kind of notification (chat, trip updates, invitations,
  general) by the event the server now names, stacked per channel.
- Location for navigator.geolocation from the fused provider, a store for
  Mapbox's map resources (30 days, 250 MB), Google Play Billing behind the
  StoreKit methods, and Google Wallet save links.
- Large screens: every orientation and window size without a restart,
  the window's size classes and a foldable's posture and hinge for the UI
  (windowLayout), the UI's keyboard shortcuts as the app's (keyCommands),
  predictive back, per-app language.
- The native strings in the 28 languages of the iOS app, generated from its
  strings files (scripts/generate-android-strings.mjs).

Debug builds carry a widget gallery that draws every widget at every size.

client/android/app/src/debug/AndroidManifest.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Debug builds only: the widget gallery (debug/WidgetGalleryActivity.kt). -->
+<manifest xmlns:android="http://schemas.android.com/apk/res/android">
+    <application>
+        <activity
+            android:name=".debug.WidgetGalleryActivity"
+            android:theme="@style/AppTheme.NoActionBar"
+            android:exported="true" />
+    </application>
+</manifest>