hub: point RTL credentials at /rtl/ and bump dev version to 0.15.12

Sovran_Bitcoin 0.15.12 serves the RTL UI under /rtl/ (upstream Angular
<base href="/rtl/"> + PathLocationStrategy); the package redirects / to
/rtl/ only for the exact root path.

Update the Hub's RTL tile so the Tor and Local Network credentials show
the canonical /rtl/ URLs (with trailing slash, which the redirect does
not cover) instead of relying on the root-path redirect. Bump the dev
versions.json fallback for rtl.service from 0.15.10 to 0.15.12 to match
the flake (deployed systems already read pkgs.sovran-bitcoin.rtl.version).

app/sovran_systemsos_web/versions.json

@@ -5,7 +5,7 @@
   "bitcoind.service": "27.1.0",
   "electrs.service": "0.10.6",
   "lnd.service": "0.18.0",
-  "rtl.service": "0.15.10",
+  "rtl.service": "0.15.12",
   "btcpayserver.service": "2.4.2",
   "albyhub.service": "1.24.0",
   "mempool.service": "3.2.1",