Add dynamic pages with clock, date, Home Assistant values and fonts

The hardcoded clock is now the built-in dynamic page "Clock". A dynamic page
has a background bitmap per time of day and up to 8 elements (clock, date,
Home Assistant value, text), each with position, alignment, font and overlay.
Pages are drawn on the ESP32, also for the web previews.

- Pages tab in the web UI: page editor with drag and drop, font upload
  (TTF/OTF converted in the browser to the FDF1 dot font format)
- Built-in fonts Small, Normal, Large and Full height (generated by
  tools/make_builtin_fonts.py), built-in bitmaps Pikachu and Pikachu yawning
- Home Assistant values over the REST API (URL and token in the HA tab)
- Settings version 4: evening settings removed (now time slots of the Clock
  page), old records are converted
- HA Mode dropdown lists the dynamic pages and bitmaps

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

src/esp32/web/page_json.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <ArduinoJson.h>
+
+#include "logic/page.h"
+
+// Dynamic pages (logic/page.h) as JSON for the web UI:
+//   {slots: [{start: "08:00", bitmap: "Pikachu"}],
+//    elements: [{type: "clock"|"date"|"ha"|"text", x, y, align: "left"|"center"|"right",
+//                overlay: "top"|"box"|"invert", font, format, language: "nl"|"en",
+//                decimals (-1 = as received), text, suffix, entity, attribute}]}
+
+void pageToJson(const DynamicPage &page, JsonObject out);
+// Returns nullptr or a message for the user. Validates the page (validatePage()).
+const char *pageFromJson(JsonVariantConst in, DynamicPage &page);