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/src/main/res/values/plurals.xml

@@ -1,17 +1,24 @@
 <?xml version="1.0" encoding="utf-8"?>
 <resources>
+    <!-- Shown on book cards and in various dialogs. %1$d = number of books. -->
     <plurals name="book_count">
         <item quantity="one">%1$d book</item>
         <item quantity="other">%1$d books</item>
     </plurals>
+
+    <!-- Word-only plural (no number prefix) used in sentences. Example: "remove 3 books from shelf". -->
     <plurals name="book_word">
         <item quantity="one">book</item>
         <item quantity="other">books</item>
     </plurals>
+
+    <!-- Shown when deleting multiple shelves. %1$d = number of shelves. -->
     <plurals name="shelf_count">
         <item quantity="one">%1$d shelf</item>
         <item quantity="other">%1$d shelves</item>
     </plurals>
+
+    <!-- Shown in the search results summary bar. %1$d = total number of results found. -->
     <plurals name="search_results_count">
         <item quantity="one">%1$d result found</item>
         <item quantity="other">%1$d results found</item>