customizable packet fragmentation settings (#9597)

* first changes

* fix: use ItemsSource for fragment packets ComboBox so value saves correctly

* feat: add customizable packet fragmentation for Xray-core

- Add fragment settings in Core tab: Packets (dropdown), Length (range), Interval (range), MaxSplit (0-10000)
- Support both 'from-to' (e.g., 50-100) and single value (e.g., 50) formats for Length, Interval, and MaxSplit
- Validation with shortened error message: 'Invalid range format. Use from-to (e.g., 50-100).'
- UI in PopupBox (materialDesign) visible only when EnableFragment is checked
- Only Xray-core supported (Mihomo/sing-box fragment code removed)
- Localization updated for 8 languages

* fix: address 2dust review feedback

- Move FragmentPacketsOptions to Global class
- Remove default values from Fragment4RayItem model (set in ConfigHandler)
- Fix Avalonia bindings: move ItemsSource and SelectedItem to code-behind
- Update ViewModel to use Global.FragmentPacketsOptions
- Remove defaults from model properties

* Adjust ui

---------

Co-authored-by: 2dust <31833384+2dust@users.noreply.github.com>

v2rayN/ServiceLib/Models/Configs/ConfigItems.cs

@@ -249,6 +249,7 @@ public class Fragment4RayItem
     public string? Packets { get; set; }
     public string? Length { get; set; }
     public string? Interval { get; set; }
+    public string? MaxSplit { get; set; }
 }
 
 [Serializable]