refactor(reader): update progress tracking with EPUB location data

Simplify and enhance reading progress updates:
- Simplify payload structure by flattening location object
- Add epubcfi field to preserve EPUB reading position
- Add percentage field for normalized progress tracking
- Remove unnecessary nested location structure

This aligns with the updated backend API and provides
better support for reflowable EPUB content position tracking.

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

@@ -1,9 +1,4 @@
-import { apiPut } from "../../api";
-
-interface ReadingProgress {
-  current_page: number;
-  total_pages: number;
-}
+import { apiPut, ReadingProgress } from "../../api";
 
 interface ExtendedProgress {
   character?: number;
@@ -20,25 +15,13 @@ export async function updateReadingProgress(
     console.warn("Skipping progress update - no valid mediaItemId");
     return;
   }
+
   const payload: any = {
-    location: {
-      page: progress.current_page,
-      total_pages: progress.total_pages,
-    },
+    current_page: progress.current_page,
+    total_pages: progress.total_pages,
+    epubcfi: progress.epubcfi || "", // ADD THIS
+    percentage: extended?.percentage || 0, // ADD THIS
   };
-
-  if (extended) {
-    if (extended.character !== undefined) {
-      payload.location.character = extended.character;
-    }
-    if (extended.chapter !== undefined) {
-      payload.location.chapter = extended.chapter;
-    }
-    if (extended.percentage !== undefined) {
-      payload.location.percentage = extended.percentage;
-    }
-  }
-
   const response = await apiPut(
     `/media-items/${mediaItemId}/progress`,
     payload,