refactor(mining): surface inconclusive benchmark; dedup idle combo

- mining_benchmark: when a thread benchmark finishes with no nonzero
  hashrate sample (e.g. the pool never reported a rate), don't finish
  silently — set a new `inconclusive` flag on ThreadBenchmarkUpdate and
  let mining_tab warn. Keeps the state-machine core pure/no-I/O (it is
  unit-tested), rather than calling the Notifications singleton from it.
- mining_controls: fold the two byte-identical idle-delay combo blocks
  (non-scaling + thread-scaling branches) into one lambda parameterised
  by combo id; removes ~30 lines of duplication.
- request_payment: drop the dead s_selected_addr_idx state and write the
  address-selected comparisons as std::string == char* consistently.

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

src/ui/windows/mining_benchmark.cpp

@@ -189,6 +189,11 @@ ThreadBenchmarkUpdate AdvanceThreadBenchmark(ThreadBenchmark& benchmark,
                         update.startPoolMining = true;
                         update.startThreads = benchmark.optimal_threads;
                     }
+                } else {
+                    // No candidate produced a nonzero hashrate (e.g. a pool that never
+                    // reported a rate). Don't finish silently — flag the run as
+                    // inconclusive so the caller can surface it (keeps this core pure).
+                    update.inconclusive = true;
                 }
             }
             break;