fix: replace invalid new(expression) calls with proper pointer allocation

Go's new() builtin takes a type and allocates a zero value — it cannot
wrap an expression. All instances of new(someExpression) were compile
errors. Replace each with a local variable assignment and address-of
operator.

Affected files:
- handlers/koreader.go: progress field pointers (Chapter, Page, etc.)
- handlers/kobo.go: pagesRemaining pointer
- handlers/queue.go: uuidPtrToString and timestamptzPtrToString helpers
- router/reader.go: bookmark pageNumber and chapterNumber pointers
- services/media_scanner.go: validation error message pointers
- services/worker.go: StartedAt and CompletedAt timestamps
- sync/offline.go: GetDeviceStatus return pointer
- tests/device_test.go: SyncEnabled and SyncFrequencyMinutes pointers

internal/services/media_scanner.go

@@ -1320,12 +1320,13 @@ func (s *MediaScanner) ValidateMediaItemForLibrary(
 		// Must be fixed-layout or comic archive
 		if mediaItem.FormatGroup != "fixed_layout" &&
 			mediaItem.FormatGroup != "comic_archive" {
-			return new(fmt.Sprintf(
+			str := fmt.Sprintf(
 				"EPUB file '%s' is reflowable (text-based), not fixed-layout (image-based). "+
 					"Manga library only accepts fixed-layout EPUBs, CBZ, CBR, or image files. "+
 					"Consider moving this file to an ebooks library.",
 				mediaItem.Title,
-			))
+			)
+			return &str
 		}
 
 		// Set manga-specific flags for fixed-layout EPUBs
@@ -1350,11 +1351,12 @@ func (s *MediaScanner) ValidateMediaItemForLibrary(
 		// Accept comic archives and fixed-layout
 		if mediaItem.FormatGroup != "comic_archive" &&
 			mediaItem.FormatGroup != "fixed_layout" {
-			return new(fmt.Sprintf(
+			str := fmt.Sprintf(
 				"File '%s' is not a comic archive format. "+
 					"Comics library only accepts CBZ, CBR, CB7, CBT, PDF, or fixed-layout EPUBs.",
 				mediaItem.Title,
-			))
+			)
+			return &str
 		}
 	}
 
@@ -1363,11 +1365,12 @@ func (s *MediaScanner) ValidateMediaItemForLibrary(
 		// Flag manga for potential reorganization (info level)
 		if mediaItem.FormatGroup == "fixed_layout" &&
 			(!mediaItem.MangaType.Valid || mediaItem.MangaType.String == "yes" || mediaItem.MangaType.String == "yes_and_right_to_left") {
-			return new(fmt.Sprintf(
+			str := fmt.Sprintf(
 				"File '%s' appears to be manga (fixed-layout with images). "+
 					"Consider moving to a manga or comics library for better organization.",
 				mediaItem.Title,
-			))
+			)
+			return &str
 		}
 	}