Add scrolling, countdown, stopwatch, images, seconds, multiplier and degree sign

- Text elements can scroll in a window at a set speed; clock, countdown and
  stopwatch can show seconds. Moving pages are sent as instant frames (new
  Nano command D1) and paced by the estimated flip time.
- Countdown to a date and time (smart, days, days + time, with seconds),
  stopwatch (web UI, HA switch and reset button), image element (fixed or
  by Home Assistant state), multiplier for HA values, degree sign (char 127)
  in all fonts and the font converter.
- Page format version 2; version 1 pages are converted.

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

src/esp32/config.h

@@ -49,6 +49,9 @@ const uint32_t POWER_OFF_DELAY = 30000;
 // timeout (a read blocks loop() for at most about twice this)
 const uint32_t HA_POLL_INTERVAL = 60000;
 const uint32_t HA_HTTP_TIMEOUT = 1500;
+// Moving dynamic pages (scrolling, seconds): sending a frame to the Nano takes about this
+// long (a bitmap at 115200 baud), plus the flip time of the changed dots
+const uint32_t FRAME_SEND_TIME = 60;
 
 // ---------------------------------------------------------------------------
 // Flashing the Nano over WiFi (see README.md, pio run -e nano_ota -t upload)
@@ -99,7 +102,7 @@ const uint32_t SETUP_AP_DELAY = 20000;        // open the AP when WiFi fails thi
 const uint32_t WIFI_CONNECT_TIMEOUT = 30000;
 const uint32_t WIFI_RESTART_TIME = 10UL * 60 * 1000;
 
-#define FIRMWARE_VERSION "2.4.0"
+#define FIRMWARE_VERSION "2.5.0"
 
 // Home Assistant device id (do not change: Home Assistant's entities are linked to it)
 const uint8_t HA_DEVICE_ID[] = {0x34, 0x86, 0x5d, 0x3a, 0x08, 0x28};