Power-cycle-friendly Nano boot and ESP relay handling

Nano / driver:
- No full on/off flip at boot; the first display() after boot is a forced
  full redraw instead, since the physical dot state is unknown
- Send READY when booted

ESP32:
- relayOn() only switches and waits when the relay was off, and waits for
  the Nano's READY (3 s timeout) instead of a fixed 3 s on every frame
- relayOff() ends Serial and releases the TX pin so the idle-high line no
  longer back-powers the Nano through its RX pin
- No display commands are sent while the relay is off
- Switching the relay on cancels a pending power-off countdown

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

src/esp32/libs.h

@@ -44,7 +44,11 @@ HASwitch powerSwitch("FlipDotDisplayPower");
 bool power = true;
 
 const int led1 = BUILTIN_LED; // Pin of the LED
-const int relay = 13; // Pin of the relay
+const int relay = 13; // Pin of the relay, powers the panels and the Nano (LOW = on)
+const int nano_tx = 1; // UART0 TX, wired to the Nano RX
+bool relay_on = false;
+// Max wait for the Nano's READY after power on (also the wait when Nano TX is not wired to our RX)
+const uint32_t NANO_BOOT_TIMEOUT = 3000;
 
 int hour = 0;
 int minute = 0;