feat(reader): add core infrastructure for feature-based architecture

- Add reader-context.ts: defines ReaderContext interface and factory
- Add reader-events.ts: event bus for feature communication
- Add reader-state.ts: centralized state management
- Add reader-navigation.ts: unified navigation and rendering API
- Add reader-services.ts: shared services (progress, chapters)
- Establishes foundation for Feature Registration Pattern

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

@@ -0,0 +1,55 @@
+import { apiPut } from "../../api";
+
+interface ReadingProgress {
+  current_page: number;
+  total_pages: number;
+}
+
+export async function updateReadingProgress(
+  mediaItemId: string,
+  progress: ReadingProgress,
+): Promise<void> {
+  const response = await apiPut(
+    `/media-items/${mediaItemId}/progress`,
+    progress,
+  );
+  await response.json();
+}
+
+export function getChapterNavigation(chapters: any[]) {
+  return {
+    getNextChapter: (currentPage: number) => {
+      for (let i = 0; i < chapters.length - 1; i++) {
+        const chapter = chapters[i];
+        const nextChapter = chapters[i + 1];
+
+        if (
+          currentPage >= chapter.start_page &&
+          currentPage < nextChapter.start_page
+        ) {
+          return nextChapter.start_page;
+        }
+      }
+      return null;
+    },
+
+    getPreviousChapter: (currentPage: number) => {
+      for (let i = 1; i < chapters.length; i++) {
+        const chapter = chapters[i];
+
+        if (
+          currentPage >= chapter.start_page &&
+          currentPage < chapter.start_page + chapter.page_count
+        ) {
+          return chapters[i - 1].start_page;
+        }
+      }
+
+      if (currentPage < chapters[0].start_page) {
+        return null;
+      }
+
+      return chapters[0].start_page;
+    },
+  };
+}