fix(app): advance logic timers by wall-clock time

Logic timers advanced by io.DeltaTime, which update() clamps to 0.25 s
so animations don't jump after a pause. That matched real time only
while the main loop rendered continuously: with frames seconds apart, a
timer fed the clamped delta runs up to 8x slow. Affected: the refresh
scheduler, the seed-migration poll, the chat fast scan, the encryption
check, the loading and shutdown timers, the lock-screen countdowns, the
Settings debug gate and the mining benchmark.

- App::logicDeltaTime(): steady-clock seconds since the previous
  update(), capped at 6 s so a sleep/resume fires each timer once
  instead of replaying the gap. Visual interpolation keeps the clamped
  io.DeltaTime.
- The refresh timers tick before the RPC results are drained, so a
  hold that a callback arms (the connect-retry backoff) starts fresh
  instead of being charged the gap before it existed.
- The shutdown timer is measured from shutdown_start_time_:
  beginShutdown runs from SDL event handling, so an accumulated delta
  would count the idle gap before the close request.
- The lock-screen error and lockout countdowns subtract only the time
  since they were armed: a failed unlock arms them from an RPC result,
  which can land on the frame that ends an idle gap.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ui/pages/settings_page.cpp

@@ -2865,7 +2865,7 @@ void RenderSettingsPage(App* app) {
         if (s_settingsState.debug_gate_open) {
             auto& st = s_settingsState;
             if (st.debug_gate_err_timer > 0.0f) {
-                st.debug_gate_err_timer -= ImGui::GetIO().DeltaTime;
+                st.debug_gate_err_timer -= app->logicDeltaTime();
                 if (st.debug_gate_err_timer <= 0.0f) st.debug_gate_err.clear();
             }
             const bool needAuth = app->debugGateRequiresAuth();

src/ui/windows/mining_tab.cpp

@@ -249,7 +249,7 @@ static void RenderMiningTabContent(App* app)
     // ================================================================
     if (s_benchmark.active()) {
         auto benchmarkUpdate = AdvanceThreadBenchmark(
-            s_benchmark, ImGui::GetIO().DeltaTime, state.pool_mining.hashrate_10s);
+            s_benchmark, app->logicDeltaTime(), state.pool_mining.hashrate_10s);
         if (benchmarkUpdate.stopPoolMining) {
             app->stopPoolMining();
         }