fix(tun): make the TUN switch reflect what the Core actually runs

The switch read enable_tun_mode while the Core was given
enable_tun_mode && !session_suppression, so falling back to Sidecar left it
showing a TUN that was not running, with nothing to write the flag back.
Drop the session flag: the stored preference is the only truth, re-checked
at startup and written off when the conditions for it are not met, and the
switch renders it against current capability.

src/components/shared/proxy-control-switches.tsx

@@ -196,12 +196,12 @@ const ProxyControlSwitches = ({
       {isTunMode && (
         <SwitchRow
           label={t('settings.sections.proxyControl.fields.tunMode')}
-          active={enable_tun_mode || false}
+          active={(enable_tun_mode && isTunModeAvailable) || false}
           infoTitle={t('settings.sections.proxyControl.tooltips.tunMode')}
           onInfoClick={() => tunRef.current?.open()}
           onToggle={handleTunToggle}
           onError={onError}
-          highlight={enable_tun_mode || false}
+          highlight={(enable_tun_mode && isTunModeAvailable) || false}
           extraIcons={
             <>
               {!isTunModeAvailable && (