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_tab.cpp

@@ -248,6 +248,11 @@ static void RenderMiningTabContent(App* app)
         if (benchmarkUpdate.startPoolMining) {
             app->startPoolMining(benchmarkUpdate.startThreads);
         }
+        if (benchmarkUpdate.inconclusive) {
+            Notifications::instance().warning(
+                "Benchmark inconclusive: no hashrate samples were recorded. "
+                "Check the pool connection and try again.");
+        }
     }
 
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