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/pages.go

@@ -75,6 +75,10 @@ func (a *App) homeExpiryOptions(settings services.UploadPolicySettings, user ser
 		unlimited = true
 	case loggedIn:
 		maxDays = a.settingsService.EffectivePolicyForUser(settings, user).MaxDays
+		// A negative per-user MaxDays override means unlimited retention.
+		if maxDays < 0 {
+			unlimited = true
+		}
 	}
 	return buildExpiryOptions(maxDays, unlimited)
 }
@@ -103,6 +107,10 @@ func buildExpiryOptions(maxDays int, unlimited bool) ([]expiryOption, int) {
 	if len(options) == 0 {
 		options = append(options, expiryOption{Minutes: capMinutes, Label: expiryLabel(capMinutes)})
 	}
+	// Unlimited uploaders can pick "never expires" (sentinel -1) after the ladder.
+	if unlimited {
+		options = append(options, expiryOption{Minutes: -1, Label: "Unlimited (never expires)"})
+	}
 
 	// Default to 24h when available, otherwise the smallest option offered.
 	defaultMinutes := options[0].Minutes
@@ -154,5 +162,9 @@ func (a *App) homeUploadPolicyLabels(settings services.UploadPolicySettings, use
 	if policy.StorageQuotaSet {
 		quota = services.FormatMegabytesLabel(policy.StorageQuotaMB)
 	}
-	return maxUpload, "Daily cap: " + services.FormatMegabytesLabel(policy.DailyUploadMB) + " · Storage quota: " + quota + " · " + strconv.Itoa(policy.MaxDays) + " day max."
+	expiryLimit := strconv.Itoa(policy.MaxDays) + " day max."
+	if policy.MaxDays < 0 {
+		expiryLimit = "no expiry limit."
+	}
+	return maxUpload, "Daily cap: " + services.FormatMegabytesLabel(policy.DailyUploadMB) + " · Storage quota: " + quota + " · " + expiryLimit
 }