refactor(reader): integrate page-based navigation into core system

Update core reader infrastructure to support new page-based navigation system
for reflowable formats while maintaining existing functionality for PDF, comic,
and manga formats.

## Core Integration Changes

### reader-context.ts
- Update imports to use new formats/reflowable module paths
- Maintain backward compatibility with existing type definitions

### reader-navigation.ts
- **Replace spine-based scrolling with page-based navigation**
- Integrate reflowable navigation modules for ebook handling
- Add imports for new navigation, progress tracking, and content rendering
- Implement discrete page navigation (no scrolling within pages)

## Navigation System Upgrade

### Previous (Broken)
- Spine-based scrolling: Scroll through entire chapters
- No page boundaries: Couldn't track position within content
- Progress tracking failed: No granular position data
- Position saving broken: Only saved chapter, not page

### New (Working)
- Page-based navigation: Discrete page boundaries
- CFI progress tracking: Precise position within content
- Position restoration: Accurate page restoration on reload
- Real pagination: Actual page numbers instead of chapter offsets

## Format Support

### Reflowable Formats (EPUB, FB2, TXT, HTML)
- Use new page-based navigation system
- Support for CFI-based progress tracking
- Proper pagination with word-count estimation
- Page content extraction and rendering

### PDF, Comic, Manga
- Maintain existing navigation functionality
- No changes to working systems
- Preserve user experience for these formats

## Technical Implementation

- ReflowableBook type casting for type safety
- Navigation functions (nextPage, previousPage, goToPage)
- Progress tracking integration
- Content rendering with page data
- UI updates for page indicators

This integration fixes the core pagination issues that prevented proper reading
progress tracking and position management for reflowable formats.

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

@@ -1,4 +1,4 @@
-import { PageCalculationResult } from "../ebook/page-calculator";
+import { PageCalculationResult } from "../formats/reflowable/page-calculator";
 
 interface UniversalReader {
   type: "ebook";