feat(proguard): IMPL_07 - Add missing Widget & Compose ProGuard rules

- Add Widget ActionCallback rules to prevent R8 removal (fixes potential widget crashes)
- Add Glance state preservation rules for widget preferences
- Add Compose TextLayout rules to prevent gradient regression in release builds
- Bump version to 1.8.1 (versionCode 21, versionName "1.8.1")

Fixes:
- Widget actions (ToggleChecklistItemAction, etc.) could be stripped by R8
- Glance widget state (DataStore-based) needs reflection protection
- Compose onTextLayout callbacks could be optimized away causing gradient issues

Files changed:
- android/app/proguard-rules.pro: Added v1.8.1 rules section (+17 lines)
- android/app/build.gradle.kts: Version bump 1.8.0 → 1.8.1

android/app/build.gradle.kts

@@ -20,8 +20,8 @@ android {
         applicationId = "dev.dettmer.simplenotes"
         minSdk = 24
         targetSdk = 36
-        versionCode = 20  // 🎉 v1.8.0: Widgets, Sorting, UI Polish, Post-Update Changelog
-        versionName = "1.8.0"  // 🎉 v1.8.0: Major Feature Release
+        versionCode = 21  // 🐛 v1.8.1: Checklist Fixes, Widget Sorting, ProGuard Audit
+        versionName = "1.8.1"  // 🐛 v1.8.1: Bugfix & Polish Release
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
     }