fix(mining): keep the thread benchmark running while the tab is hidden

The thread benchmark state machine was advanced from the Mining tab's
render path, so it only progressed while that tab was visible and the
window was on screen. Hiding the tab or minimizing the window froze the
benchmark mid-sweep.

- The benchmark now advances from the main update loop, ungated by
  connectivity, so it keeps running when the tab is hidden or the window
  is minimized.
- Its clock is driven from a steady wall-clock reading rather than the
  render delta, and each step is clamped like the other logic timers so
  a suspend/resume cannot leap a benchmark phase.

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

src/ui/windows/mining_tab.h

@@ -28,5 +28,11 @@ bool IsMiningBenchmarkActive();
  */
 void CancelMiningBenchmark(App* app, bool restoreMiner = true);
 
+/**
+ * @brief Advance the thread benchmark state machine. Call once per App::update() (ungated by connectivity).
+ * No-op when no benchmark is active.
+ */
+void TickMiningBenchmark(App* app, double hashrate10s);
+
 } // namespace ui
 } // namespace dragonx