Fix chromecast support

[nerijus]

In internal/device/device.go, Discover() only does an SSDP search for urn:schemas-upnp-org:device:MediaRenderer:1. Chromecasts don't advertise via SSDP as MediaRenderers — they announce over mDNS as _googlecast._tcp.local. There's even a comment admitting it:

"Chromecast discovery requires mDNS which the current dependency set doesn't ship; callers connect to known Chromecast addresses directly."

That comment is actually outdated: castor already depends on github.com/vishen/go-chromecast v0.3.4 for playback, and that same module ships a dns package with DiscoverCastDNSEntries() built on grandcat/zeroconf — whose hashes are already in castor's go.sum. So no new dependencies are needed at all.
The patch does three things:

internal/device/device.go — Discover() now runs the SSDP scan and an mDNS _googlecast._tcp browse in parallel under the same timeout, merging results. The old body moved into discoverDLNA().
internal/device/chromecast.go — adds discoverChromecast(), which drains the entry channel until the timeout, dedupes by UUID (mDNS re-announces duplicates), prefers the friendly DeviceName from the TXT record, and preserves non-default ports in the address as host:port — cast groups advertise on random high ports, not 8009.
connectChromecast() — now parses an optional host:port address via net.SplitHostPort, falling back to 8009 for a bare host, so existing configs with plain IPs keep working and cast groups become connectable.
go.mod — adds the four indirect requires (grandcat/zeroconf, miekg/dns, cenkalti/backoff, golang.org/x/net); all already present in go.sum.

[stupside]

Thanks a lot for this. I combined it with #19 into #21, where your discovery implementation is the base commit (kept under your authorship) plus a test and a DLNA cleanup on top. Closing in favor of #21. 🙏