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/kobo.go

@@ -303,7 +303,8 @@ func (h *KoboHandler) Initialization(c *echo.Context) error {
 				lastModified = progress.LastReadAt.Time.Format(time.RFC3339)
 			}
 			if progress.TotalPages.Valid && progress.CurrentPage.Valid {
-				pagesRemaining = new(int(progress.TotalPages.Int32 - progress.CurrentPage.Int32))
+				remaining := int(progress.TotalPages.Int32 - progress.CurrentPage.Int32)
+				pagesRemaining = &remaining
 			}
 		}