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/dashboard.go

@@ -82,7 +82,7 @@ func (a *App) Dashboard(w http.ResponseWriter, r *http.Request) {
 			FileCount:      len(row.Box.Files),
 			Size:           row.TotalSizeLabel,
 			CreatedAt:      row.Box.CreatedAt.Format("Jan 2 15:04"),
-			ExpiresAt:      row.Box.ExpiresAt.Format("Jan 2 15:04"),
+			ExpiresAt:      boxExpiryLabel(row.Box.ExpiresAt, "Jan 2 15:04"),
 			URL:            "/d/" + row.Box.ID,
 		})
 	}