feat: initialize page calculation on reader load

- Add pageCalculationResult and currentScrollPosition to UniversalReader type
- Add settings:changed event type for settings change notifications
- Call initializePageCalculation after reader ready in reader-shell.ts
- Add tabindex to reader-content for keyboard navigation focus

web/src/reader/core/reader-context.ts

@@ -1,4 +1,5 @@
 import type { ReaderMetadata } from "../../types/reader";
+import { PageCalculationResult } from "../ebook/page-calculator";
 
 type ReaderType = "ebook" | "pdf" | "comic" | "manga";
 
@@ -6,6 +7,9 @@ interface UniversalReader {
   type: "ebook";
   cif: any;
   currentSpineIndex: number;
+  currentPage?: number;
+  pageCalculationResult?: PageCalculationResult;
+  currentScrollPosition?: number;
 }
 
 interface PDFReader {