fix(send): restart the fast-lane worker on reconnect so the opid poll runs

A completed send could spin forever on "Waiting for operation (N)". Root
cause: onDisconnected() stopped fast_worker_ but kept the unique_ptr, so
onConnected()'s `if (!fast_worker_)` guard never restarted it — after the
first reconnect (daemon warmup, restart, any RPC blip) the fast lane stayed
dead for the whole session.

The opid poll was the only fast_worker_ user that posted to it directly with
no fallback, so it alone broke: its post() landed on a stopped thread, the
result MainCb never ran, opid_poll_in_progress_ stuck true, and the poll never
fired again — leaving the operation (already "success" on the daemon, with a
txid) untracked.

Two fixes:
- onDisconnected() now reset()s fast_worker_ after stop(), so onConnected
  recreates and starts a fresh one (restores the fast lane for all its users,
  not just the poll).
- the opid poll now falls back to worker_ when the fast lane isn't running,
  matching every other fast_worker_ call site — defense in depth.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app.cpp

@@ -760,13 +760,18 @@ void App::update()
         }
     }
 
-    // Poll pending z_sendmany operations for completion (full-node opid flow; lite has none)
+    // Poll pending z_sendmany operations for completion (full-node opid flow; lite has none).
+    // Prefer the fast lane but fall back to the main worker (mirrors every other fast_worker_ user)
+    // so a torn-down/not-yet-restarted fast lane can't silently strand the poll on "Waiting for
+    // operation" — the symptom when fast_worker_ was stopped on reconnect and never came back.
+    rpc::RPCWorker* opidWorker = (fast_worker_ && fast_worker_->isRunning())
+                                     ? fast_worker_.get() : worker_.get();
     if (network_refresh_.isDue(RefreshTimer::Opid) && !pending_opids_.empty()
-        && rpcConnected && fast_worker_ && !opid_poll_in_progress_) {
+        && rpcConnected && opidWorker && !opid_poll_in_progress_) {
         network_refresh_.reset(RefreshTimer::Opid);
         auto opids = pending_opids_;  // copy for worker thread
         opid_poll_in_progress_ = true;
-        fast_worker_->post([this, opids]() -> rpc::RPCWorker::MainCb {
+        opidWorker->post([this, opids]() -> rpc::RPCWorker::MainCb {
             auto* rpc = (fast_rpc_ && fast_rpc_->isConnected()) ? fast_rpc_.get() : rpc_.get();
             if (!rpc) return [this](){ opid_poll_in_progress_ = false; };
             json ids = json::array();