fix: [Port Forwarding] newly added port range rule incorrectly appears in single port tab

[PeterJhongLinksys]

Description

When adding a new rule from the Port Range tab, the rule appears in the Single Port tab instead of the Port Range tab.

Root Cause

The tab filtering logic in lib/page/port_forwarding/views/usp_port_forwarding_detail_view.dart uses the isSinglePort getter from lib/page/_shared/models/port_forwarding_rule_ui_model.dart:

bool get isSinglePort =>
    externalPortEndRange == 0 || externalPortEndRange == externalPort;

If externalPortEndRange equals externalPort (e.g., when the user enters the same value for start and end), the rule is classified as single port. Additionally, if the dialog result does not correctly set externalPortEndRange, the default value of 0 would also cause the rule to be classified as single port.

Expected Behavior

Rules created from the Port Range tab should always appear in the Port Range tab. The classification logic should account for the creation context, or the dialog should enforce that end port is strictly greater than start port with a clear error message.

[PeterJhongLinksys]

[Root Cause]
The isSinglePort getter classifies a rule as single port when externalPortEndRange == 0 || externalPortEndRange == externalPort. When the port range dialog allowed end port equal to start port, or when fields were empty (defaulting to 0), rules created from the Port Range tab were misclassified as single port.

[Solution]
Fixed as part of #755. The port range dialog now enforces:

• All port fields are required (cannot be empty, so no default 0)
• End port must be strictly greater than start port

This ensures externalPortEndRange is always greater than externalPort, so isSinglePort correctly returns false for port range rules.

Commit: 8d3673d (same as #755)