fix(release): keep Room DB impls so R8 doesn't crash startup

The minified release build crashed on every launch before any UI:

  Unable to get provider androidx.startup.InitializationProvider:
    Failed to create an instance of androidx.work.impl.WorkDatabase

The home-screen widgets use Glance, which pulls in WorkManager and its
transitive Room database (room-runtime 2.2.5). Room 2.2.5's bundled keep
rule is `-keep class * extends androidx.room.RoomDatabase` — it keeps the
class but not its constructor. Under R8 full mode (AGP 9) the generated
WorkDatabase_Impl was reduced to a non-instantiable class, so Room's
reflective newInstance() threw InstantiationException at startup.

Add `-keep class * extends androidx.room.RoomDatabase { *; }` so the
generated *_Impl classes keep their constructors. Verified against the
rebuilt release APK: WorkDatabase_Impl is now PUBLIC FINAL with its
<init> present.

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

CHANGELOG.md

@@ -20,6 +20,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   anything Calendula can't represent (changed recurring occurrences, guest
   lists) is reported rather than silently dropped.
 
+### Fixed
+- Fixed the app crashing immediately on every launch in the optimized release
+  build: release code-shrinking (R8) was stripping a database class the
+  home-screen widget framework needs, so the app died at startup before showing
+  anything. Added the missing keep rule.
+
 ## [2.6.0] — 2026-06-18
 
 ### Added