perf(v1.8.0): optimize ProGuard/R8 rules for smaller APK size

- Replace broad '-keep class dev.dettmer.simplenotes.** { *; }' with targeted rules
- Keep only Gson data models and enum values that require reflection
- Allow R8 to shrink/obfuscate remaining app code
- Update rules for: Note, ChecklistItem, DeletionTracker, BackupData models
- Keep enum values() and valueOf() for state serialization
- Remove unnecessary keep rules for non-reflection classes
- Add structured comments for maintainability

APK size reduced from 5.0 MB → 4.8 MB (200 KB saved).
R8 now has better optimization opportunities while maintaining functionality.
All unit tests pass, lint reports clean (0 errors, 0 warnings).

Build: BUILD SUCCESSFUL - 4.8M app-fdroid-release.apk

Author: inventory69

android/app/proguard-rules.pro

@@ -59,8 +59,25 @@
 -keep class * implements com.google.gson.JsonSerializer
 -keep class * implements com.google.gson.JsonDeserializer
 
-# Keep your app's data classes
--keep class dev.dettmer.simplenotes.** { *; }
+# ═══════════════════════════════════════════════════════════════════════
+# App-specific rules: Only keep what Gson/reflection needs
+# ═══════════════════════════════════════════════════════════════════════
+
+# Gson data models (serialized/deserialized via reflection)
+-keep class dev.dettmer.simplenotes.models.Note { *; }
+-keep class dev.dettmer.simplenotes.models.Note$NoteRaw { *; }
+-keep class dev.dettmer.simplenotes.models.ChecklistItem { *; }
+-keep class dev.dettmer.simplenotes.models.DeletionRecord { *; }
+-keep class dev.dettmer.simplenotes.models.DeletionTracker { *; }
+-keep class dev.dettmer.simplenotes.backup.BackupData { *; }
+-keep class dev.dettmer.simplenotes.backup.BackupResult { *; }
+
+# Keep enum values (used in serialization and widget state)
+-keepclassmembers enum dev.dettmer.simplenotes.** {
+    <fields>;
+    public static **[] values();
+    public static ** valueOf(java.lang.String);
+}
 
 # v1.7.1: Suppress TextInclusionStrategy warnings on older Android versions
 # This class only exists on API 35+ but Compose handles the fallback gracefully