v1.2.10: proxy Stop actually stops now (fix issue #99)

Single-focus release. The Stop button in the UI previously only
stopped new connections from being accepted — in-flight clients kept
running on the old DomainFronter, which meant:

- Pages kept loading after Stop (users thought they'd stopped)
- Auth-key changes didn't take effect for domains with a live
  keep-alive to the proxy
- Apps Script quota could still be consumed post-Stop

Fix (7338e76): wrap per-client spawns in a tokio::task::JoinSet
inside each accept loop. On shutdown, aborting the accept task drops
the JoinSet, which aborts every in-flight client. Sockets close,
the old fronter's TLS pool drops, and a subsequent Start builds a
clean new state.

Finding 1 of #99 (quota-exceeded → "timeout" instead of the real
502 body) is a separate pool-staleness issue and is NOT addressed
in this release.

Cargo.lock

@@ -2186,7 +2186,7 @@ dependencies = [
 
 [[package]]
 name = "mhrv-rs"
-version = "1.2.8"
+version = "1.2.10"
 dependencies = [
  "base64 0.22.1",
  "bytes",