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/sync/offline.go

@@ -226,7 +226,8 @@ func (d *OfflineDetector) GetDeviceStatus(ctx context.Context, deviceID pgtype.U
 		return nil, err
 	}
 
-	return new(d.getDeviceStatus(device)), nil
+	deviceStatus := d.getDeviceStatus(device)
+	return &deviceStatus, nil
 }
 
 func (d *OfflineDetector) ListOfflineDevices(ctx context.Context) ([]DeviceOnlineStatus, error) {