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/main/res/values-fr/strings.xml

@@ -705,12 +705,15 @@
     <string name="options_language">Langue</string>
     <string name="options_export_logs_last_lines">Exporter les logs (dernières %1$d lignes)</string>
     <string name="dialog_strict_file_filter_desc">Si vous activez cette option, certains formats supportés comme AZW3, CB7 et FB2 pourraient ne pas apparaitre, selon votre gestionnaire de fichiers.\n\nÊtes-vous sûr de vouloir activer ce filtre ?</string>
+    <string name="language_system_default">Langue du système</string>
+    <string name="language_english">English (Anglais)</string>
     <string name="language_english_default">English (Anglais ; par défaut)</string>
     <string name="language_arabic">العربية (Arabe)</string>
     <string name="language_german">Deutsch (Allemand)</string>
     <string name="language_turkish">Türkçe (Turque)</string>
     <string name="language_french">Français</string>
     <string name="language_russian">Русский (Russe)</string>
+    <string name="language_spanish">Español (Espagnol)</string>
     <string name="app_theme_contrast">Contraste</string>
     <string name="app_theme_text_brightness">Luminosité du texte</string>
     <string name="app_theme_preset_ocean">Océan</string>