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/handlers/queue.go

@@ -310,7 +310,8 @@ func uuidPtrToString(u pgtype.UUID) *string {
 	if !u.Valid {
 		return nil
 	}
-	return new(uuid.UUID(u.Bytes).String())
+	s := uuid.UUID(u.Bytes).String()
+	return &s
 }
 
 func textPtrToString(t pgtype.Text) *string {
@@ -324,5 +325,6 @@ func timestamptzPtrToString(t pgtype.Timestamptz) *string {
 	if !t.Valid {
 		return nil
 	}
-	return new(t.Time.Format("2006-01-02T15:04:05Z07:00"))
+	timeFormat := t.Time.Format("2006-01-02T15:04:05Z07:00")
+	return &timeFormat
 }