v1.0.5: Fix history viewer - ContentConverter JsonElement type bug

- ContentConverter.tipTapToHtml() had same JsonObject.asMap() bug as tiptapToMarkdown
- All as? String casts silently returned null, producing empty HTML
- Now uses Gson TypeToken<Map<String, Any?>> for proper deserialization
- Added contentToHtml() that handles String (HTML), Map (TipTap), and JSON string
- Fixed heading level cast (Gson deserializes numbers as Double, not Int)
- VersionViewerActivity handles all content types correctly

app/build.gradle.kts

@@ -20,8 +20,8 @@ android {
         applicationId = "com.docmost.app"
         minSdk = 26
         targetSdk = 34
-        versionCode = 5
-        versionName = "1.0.4"
+        versionCode = 6
+        versionName = "1.0.5"
     }
 
     signingConfigs {