fix: derive launch port from URL authority, not naive rsplit

reachable_lan_address() parsed the launch port with url.rsplit(':')
which yields "8096/" for manifest interfaces.main URLs that carry a
path (http://localhost:8096/). That fails to parse and silently drops
a perfectly reachable launch URL, so apps like jellyfin, btcpay-server,
fedimint, gitea, nextcloud and portainer showed running with no launch
link in the UI. New launch_url_port() reads digits after the final
colon (mirroring port_from_url in the RPC layer) and tolerates a
trailing path. Adds regression tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/Cargo.lock

@@ -80,7 +80,7 @@ checksum = "a23eb6b1614318a8071c9b2521f36b424b2c83db5eb3a0fead4a6c0809af6e61"
 
 [[package]]
 name = "archipelago"
-version = "1.7.89-alpha"
+version = "1.7.90-alpha"
 dependencies = [
  "anyhow",
  "archipelago-container",