feat: auto recursive mode, fix Linux install

Auto mode (new default): probes a root server on startup; uses
recursive resolution if outbound DNS works, falls back to Quad9 DoH
if blocked. Dashboard shows mode indicator (green/yellow).

Linux install fixes:
- Add DNSStubListener=no to resolved drop-in (frees port 53)
- Configure DNS before starting service (correct ordering)
- Skip 127.0.0.53 in upstream detection
- `numa install` now does everything (service + DNS + CA)
- `numa uninstall` mirrors install (stop service + restore DNS)
- Extract is_loopback_or_stub() for consistent filtering

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

install.sh

@@ -70,8 +70,10 @@ echo ""
 echo "  \033[38;2;107;124;78mInstalled:\033[0m $INSTALL_DIR/numa ($TAG)"
 echo ""
 echo "  Get started:"
-echo "    sudo numa                    # start the DNS server"
-echo "    sudo numa install            # set as system DNS"
-echo "    sudo numa service start      # run as persistent service"
-echo "    open http://localhost:5380    # dashboard"
+echo "    sudo numa install            # install service + set as system DNS"
+echo "    open http://localhost:5380   # dashboard"
+echo ""
+echo "  Other commands:"
+echo "    sudo numa                    # run in foreground (no service)"
+echo "    sudo numa uninstall          # restore original DNS"
 echo ""