fix: stop killing NM's dnsmasq - use dnsmasq-shared.d instead

Killing all dnsmasq processes caused NetworkManager to detect its
hotspot dnsmasq died and tear down the hotspot (AP appears briefly
then disappears).

Now leverage NM's built-in dnsmasq-shared.d config directory:
write DNS hijack config before starting AP so NM's own dnsmasq
picks it up. No separate dnsmasq process needed.