Storage: poll the SD card on its own task

A failed mount retries for ~2.5 s, which froze the UI every 15 s while
no card was inserted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

src/main.cpp

@@ -35,7 +35,7 @@ static void draw() {
     d.setTextColor(TFT_WHITE);
 
     const auto& b = battery->estimator();
-    const auto& s = storage->state();
+    const auto s = storage->state();
     int y = 16;
     auto line = [&](const String& text) {
         d.drawString(text, 4, y);
@@ -122,7 +122,7 @@ void loop() {
 
     if (millis() - lastLog > 5000) {
         lastLog = millis();
-        const auto& s = storage->state();
+        const auto s = storage->state();
         Serial.printf("status: batt %d%% %dmV raw %d | sd %s %d%% total %lluMB | heap %u min %u | screen %d\n",
                       battery->estimator().percent(), battery->estimator().millivolts(),
                       battery->lastRawMillivolts(), s.present ? "yes" : "no", s.usedPercent,