feat(tun): expose mihomo tun dns-mode in TUN settings

Support the dns-mode field introduced in mihomo v1.19.33. A
button group below the stack selector offers
Auto/Disabled/Native/Hijack; Auto writes an empty string, which
the core treats as the legacy behavior. Adding dns-mode to
GUI_KEYS makes the value GUI-authoritative once saved, matching
the other TUN dialog fields.

src-tauri/src/constants.rs

@@ -75,6 +75,7 @@ pub mod tun {
         "auto-redirect",
         "auto-detect-interface",
         "dns-hijack",
+        "dns-mode",
         "strict-route",
         "mtu",
     ];