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

desktopApp/src/desktopMain/kotlin/com/aryan/reader/desktop/DesktopEpubPagination.kt

@@ -0,0 +1,65 @@
+package com.aryan.reader.desktop
+
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.material3.CircularProgressIndicator
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+import com.aryan.reader.shared.reader.ReaderLayoutSignature
+import com.aryan.reader.shared.reader.ReaderViewportSpec
+import com.aryan.reader.shared.reader.SharedEpubBook
+
+internal data class DesktopEpubPaginationRequest(
+    val bookId: String,
+    val chapterSignature: Int,
+    val layoutSignature: ReaderLayoutSignature,
+    val viewport: ReaderViewportSpec,
+    val density: DesktopEpubPaginationDensity,
+    val cacheGeneration: Int
+)
+
+internal data class DesktopEpubPaginationDensity(
+    val density: Float,
+    val fontScale: Float
+)
+
+internal fun SharedEpubBook.desktopPaginationContentSignature(): Int {
+    return chapters.fold(31 * id.hashCode() + css.hashCode()) { acc, chapter ->
+        31 * acc +
+            chapter.id.hashCode() +
+            chapter.plainText.length +
+            chapter.plainText.hashCode() +
+            chapter.semanticBlocks.hashCode() +
+            chapter.htmlContent.length +
+            chapter.htmlContent.hashCode() +
+            chapter.baseHref.orEmpty().hashCode()
+    }
+}
+
+@Composable
+internal fun DesktopEpubPaginationPreparing(
+    active: Boolean,
+    modifier: Modifier = Modifier
+) {
+    Box(
+        modifier = modifier,
+        contentAlignment = Alignment.Center
+    ) {
+        Column(
+            horizontalAlignment = Alignment.CenterHorizontally,
+            verticalArrangement = Arrangement.spacedBy(12.dp)
+        ) {
+            CircularProgressIndicator()
+            Text(
+                if (active) "Preparing pages" else "Measuring reader layout",
+                style = MaterialTheme.typography.bodyMedium,
+                color = MaterialTheme.colorScheme.onSurfaceVariant
+            )
+        }
+    }
+}