fix: Restore normal CPU frequency before drawing sleep screen

When auto-sleep triggers after inactivity, the CPU remains at 10 MHz
(low power mode) during sleep screen rendering, causing it to draw
much slower than expected.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/main.cpp

@@ -210,6 +210,7 @@ void waitForPowerRelease() {
 
 // Enter deep sleep mode
 void enterDeepSleep() {
+  powerManager.setPowerSaving(false);
   APP_STATE.lastSleepFromReader = currentActivity && currentActivity->isReaderActivity();
   APP_STATE.saveToFile();
   exitActivity();