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>

CLAUDE.md

@@ -88,7 +88,8 @@ See README.md for the overview.
 - Frames are only sent when the screen changes. The schedule uses 08:00 as the start of
   the day on purpose (that is what the old code did, and the README documents it).
 - Home Assistant switches are not optimistic: callbacks must call `sender->setState()`.
-  The unique ids `FlipDotDisplayClock` / `FlipDotDisplayPower` and `HA_DEVICE_ID` must stay
+  The unique ids (`FlipDotDisplayPower`, `FlipDotDisplayMode`, `FlipDotDisplayOffImage`,
+  `FlipDotDisplayStopwatch`, ...) and `HA_DEVICE_ID` must stay
   the same, or Home Assistant creates new entities.
 
 ## Keeping ESP32 OTA alive (most important requirement)
@@ -144,6 +145,10 @@ See README.md for the overview.
   previews, so there is one renderer. Pages and bitmaps share one namespace (checked in
   `app.cpp`); things a page uses can't be deleted. The built-in page "Clock" replaces the
   old hardcoded clock (also the default and the HA "Clock" option).
+- Moving pages: `renderPage()` returns when the page changes next (scroll step, second);
+  `FlipdotFrames` sends those frames as `D1` (no effect) and paces them by the estimated
+  flip time, because the Nano has no acknowledgement. Character 127 is the degree sign in
+  page texts and fonts (UTF-8 ° in JSON).
 - Fonts use the "FDF1" format of `logic/font.h`; the web page writes the same format.
   `logic/builtin_fonts.cpp` is generated: change `tools/make_builtin_fonts.py` and rerun it.
 - Home Assistant values come from HA's REST API (`net/ha_api`), one blocking request per