String extraction (#174)

* Refactored the EPUB reader to use localized string resources instead of hardcoded strings across multiple components.

* Refactored the PDF viewer and annotation components to use centralized string resources instead of hardcoded text.

* Updated `strings.xml` with extensive translation hints and localization metadata.

app/build.gradle.kts

@@ -40,6 +40,7 @@ android {
             }
         }
         buildConfigField("boolean", "IS_PRO", "false")
+        buildConfigField("boolean", "IS_OFFLINE", "false")
     }
 
     flavorDimensions += "version"
@@ -91,6 +92,12 @@ android {
                 "proguard-rules.pro"
             )
         }
+
+        create("releaseOffline") {
+            initWith(getByName("release"))
+            matchingFallbacks += listOf("release")
+            buildConfigField("boolean", "IS_OFFLINE", "true")
+        }
     }
 
     applicationVariants.all {