feat(templates): expand reader and progress template types for format-aware display

Add fields to ReaderMetadata and ReadingProgress template types to support
KOReader-like progress display:

ReaderMetadata:
- TotalCharacters: from media item, used for estimated page calculation
- EstimatedPages: computed via sync.EstimatedPages()

ReadingProgress:
- Chapter: current chapter index from reading_progress
- ChapterProgress: within-chapter progress (0-100, multiplied from DB fraction)
- FormatGroup: item format for conditional display logic

These fields enable format-aware progress display (pages for comics/PDFs,
estimated pages for reflowable, percentage for all).

templates/types.go

@@ -161,6 +161,8 @@ type ReaderMetadata struct {
 	ReadingDirection string `json:"reading_direction"`
 	FileURL          string `json:"file_url"`
 	LibraryID        string `json:"library_id"`
+	TotalCharacters  int64  `json:"total_characters"`
+	EstimatedPages   int    `json:"estimated_pages"`
 }
 
 type ReadingProgress struct {
@@ -172,6 +174,9 @@ type ReadingProgress struct {
 	Percentage      float64
 	EpubCfi         string
 	LastReadAt      time.Time
+	Chapter         int
+	ChapterProgress float64
+	FormatGroup     string
 }
 
 type Bookmark struct {