fix(calendar): don't register calendar observer before permission granted

The repository registers a ContentObserver on the calendar provider eagerly
in its init block, and an activity-scoped SettingsViewModel (which drives the
theme) injects that repository — so the @Singleton is constructed at launch,
above RootScreen's permission gate. On newer Android, registering an observer
on a provider you lack permission for throws SecurityException instead of
silently no-op'ing, so the app crashed instantly on every launch whenever
calendar access wasn't granted (fresh install or revoked permission), before
the permission screen could ever appear.

Guard the registration behind a calendar-permission check and re-attach the
observer lazily on the first calendars()/instances() read, which runs once the
gate opens and screens subscribe. Access to the observer collections is now
synchronized since registration can happen on the main thread (repo init) or
the IO dispatcher (query re-attach).

Verified on-device: permission-denied launch shows the permission screen
instead of crashing; granting it proceeds to the calendar with live updates.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/build.gradle.kts

@@ -28,8 +28,8 @@ android {
         // the tag, with versionCode = MAJOR*10000 + MINOR*100 + PATCH
         // (e.g. v2.0.0 -> 20000). These committed values are the dev/local
         // default; keep them matching the latest released tag. See docs/RELEASING.md.
-        versionCode = 20700
-        versionName = "2.7.0"
+        versionCode = 20701
+        versionName = "2.7.1"
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
     }