General improvements (#316)

* Added support for right text alignment in epub reader

* Added tabs management to PDF navigation drawer

* Implemented unified selection menu placement logic

* Added reset functionality for toolbar customization

* Improved highlight filtering in epub pagination

* Migrated hardcoded UI strings to string resources

* Extracted desktop application logic from Main.kt into modular files

* Refactored Main.kt by extracting PDF and EPUB logic into specialized files

* Added Spanish language support

* Added system default option to app language selection

app/src/test/java/com/aryan/reader/AppLanguageOptionsTest.kt

@@ -0,0 +1,60 @@
+package com.aryan.reader
+
+import java.io.File
+import javax.xml.parsers.DocumentBuilderFactory
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class AppLanguageOptionsTest {
+
+    @Test
+    fun `supported app languages include Spanish`() {
+        assertEquals(
+            listOf("en", "ar", "de", "tr", "fr", "ru", "es"),
+            supportedAppLanguageOptions.mapNotNull { it.tag }
+        )
+        assertEquals(R.string.language_spanish, supportedAppLanguageOptions.last().labelRes)
+    }
+
+    @Test
+    fun `app language selection defaults to system before explicit overrides`() {
+        assertEquals(null, appLanguageSelectionOptions.first().tag)
+        assertEquals(R.string.language_system_default, appLanguageSelectionOptions.first().labelRes)
+        assertEquals(supportedAppLanguageOptions, appLanguageSelectionOptions.drop(1))
+    }
+
+    @Test
+    fun `supported app language tags are unique`() {
+        val tags = supportedAppLanguageOptions.mapNotNull { it.tag }
+
+        assertEquals(tags.distinct(), tags)
+    }
+
+    @Test
+    fun `supported app languages match Android locale config`() {
+        assertEquals(readLocaleConfigTags(), supportedAppLanguageOptions.map { it.tag })
+    }
+
+    private fun readLocaleConfigTags(): List<String> {
+        val localeConfig = listOf(
+            File("src/main/res/xml/locales_config.xml"),
+            File("app/src/main/res/xml/locales_config.xml")
+        ).first { it.isFile }
+        val document = DocumentBuilderFactory.newInstance()
+            .apply { isNamespaceAware = true }
+            .newDocumentBuilder()
+            .parse(localeConfig)
+        val localeNodes = document.getElementsByTagName("locale")
+
+        return buildList {
+            for (index in 0 until localeNodes.length) {
+                add(
+                    localeNodes.item(index)
+                        .attributes
+                        .getNamedItemNS("http://schemas.android.com/apk/res/android", "name")
+                        .nodeValue
+                )
+            }
+        }
+    }
+}