feat(config): allow -1 to represent unlimited upload limits

Introduce support for configuring unlimited upload limits by allowing -1
as a valid value for anonymous and user upload MB limits.

Changes include:
- Added `envMegabytesLimitFloat` and helper functions to parse and validate limits where -1 is allowed.
- Updated validation logic to accept -1 for `AnonymousMaxUploadMB`, `AnonymousDailyUploadMB`, and `UserDailyUploadMB`.
- Added a test case to verify unlimited upload policy behavior.

backend/libs/handlers/pages.go

@@ -66,7 +66,9 @@ func (a *App) homeUploadPolicyLabels(settings services.UploadPolicySettings, use
 	}
 	policy := a.settingsService.EffectivePolicyForUser(settings, user)
 	maxUpload := a.uploadService.MaxUploadSizeLabel()
-	if policy.MaxUploadMB > 0 {
+	if policy.MaxUploadMB < 0 {
+		maxUpload = "unlimited"
+	} else if policy.MaxUploadMB > 0 {
 		maxUpload = services.FormatMegabytesLabel(policy.MaxUploadMB)
 	}
 	quota := "unlimited"