Fix NPE with no active network interfaces and HostnameFetcher Java 17+ reflection fallback

[Copilot]

Two independent crash bugs, both caused by implicit assumptions that break in edge-case environments.

NPE when all interfaces are DOWN (#340)

InetAddressUtils.getLocalInterface() returns null when no non-loopback interface with a hardware address is up. AbstractFeederGUI.asyncFillLocalHostInfo() dereferenced it unconditionally in both the UnknownHostException catch block and the asyncExec callback.

// Before
catch (UnknownHostException e) {
    localName = localInterface.getAddress().getHostName(); // NPE if null
}
// asyncExec
if ("".equals(ipText.getText())) {
    ipText.setText(localInterface.getAddress().getHostAddress()); // NPE if null
}

// After
catch (UnknownHostException e) {
    localName = localInterface != null ? localInterface.getAddress().getHostName() : "localhost";
}
if ("".equals(ipText.getText()) && localInterface != null) {
    ipText.setText(localInterface.getAddress().getHostAddress());
    afterLocalHostInfoFilled(localInterface);
}

HostnameFetcher silent NPE on Java 17+

On Java 17+, the HostnameFetcher static initializer fails with InaccessibleObjectException (module system blocks InetAddress.impl), leaving getHostByAddr = null. resolveWithRegularDNS() relied on the subsequent NPE being swallowed by a broad catch (Exception e) to fall through to getCanonicalHostName(). Replaced with an explicit if (getHostByAddr != null) guard so the fallback is unconditional and the exception handling only covers the intended InvocationTargetException/UnknownHostException path.