feat(policy): support unlimited values in user policies and box expiry

- Update user policy and user update handlers to accept -1 as an unlimited value for MaxDays, DailyBoxes, ActiveBoxes, and ShortWindowRequests.
- Introduce `optionalIntAllowUnlimited` helper and update `optionalMBAllowZero` to support -1.
- Use `boxExpiryLabel` helper across admin, dashboard, and download handlers to properly format expiration dates, supporting boxes that never expire.

backend/libs/handlers/manage.go

@@ -82,7 +82,7 @@ func (a *App) managePageData(box services.Box, token string) managePageData {
 		Token:           token,
 		FileCount:       len(box.Files),
 		TotalSize:       helpers.FormatBytes(totalSize),
-		ExpiresLabel:    box.ExpiresAt.Format("Jan 2, 2006 15:04 MST"),
+		ExpiresLabel:    boxExpiryLabel(box.ExpiresAt, "Jan 2, 2006 15:04 MST"),
 		DownloadCount:   box.DownloadCount,
 		MaxDownloads:    box.MaxDownloads,
 		Protected:       a.uploadService.IsProtected(box),