Home Assistant requests in a task, progress bar, Plex page, bitmap cache

- net/ha_api: the HTTP requests run in a FreeRTOS task on core 0; loop()
  only hands over copies through queues. The web UI's test button polls.
  No more 3-4 s loop() stalls once a minute; timeout raised to 5 s.
- Progress bar page element (page format 4, v3 pages are converted)
- Plex user setting (settings v6): {plex_user} in entity ids
- Plex page (examples): watchers, play/pause icon, title, progress
- Bitmaps cached in RAM: a LittleFS read took ~100 ms on every render
  and web preview; long JSON lists sent in 2 KB pieces
- Backup includes the refresh settings and the Plex user

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

docs/web-ui.md

@@ -36,9 +36,11 @@ arrow keys. Built-in pages can be copied, not changed. See the [pictures of the
 | Countdown | To a date and time: smart (`12 dagen`, `5 uur`, `34 min`, `0:45`), days, `12d 04:05`, with seconds; a text when it has passed |
 | Stopwatch | `4:05`, `0:04`, `0:04:05`; started in the [Stopwatch](#stopwatch) tab or Home Assistant |
 | Image | A bitmap at a position, fixed or chosen by a Home Assistant state (`weer ` + `cloudy` = the bitmap `weer cloudy`) |
+| Progress bar | Filled from the left by a Home Assistant number: value x multiplier / "full at" (a percentage: full at 100). Width, height, with or without a frame |
 
 Every element has a position, alignment, font and overlay (on top, box that clears the
-dots behind it, inverted). Texts can scroll in a window at a chosen speed and can contain
+dots behind it, inverted). Entity ids can contain `{plex_user}`, replaced by the Plex user
+of the Home Assistant tab (e.g. `sensor.{plex_user}_state`). Texts can scroll in a window at a chosen speed and can contain
 the degree sign. Fonts: Small (3x5), Normal (5x7), Large (5x7 doubled), Full height (19 dot
 digits), and your own: upload a TTF/OTF, choose the height in dots, the browser converts
 it.
@@ -98,8 +100,8 @@ switches either off); test patterns; the status LED.
 
 MQTT: on/off, broker, port, user, password (changing these restarts the ESP32; an empty
 broker uses `secrets.h`). The URL and long-lived access token for the values on dynamic
-pages and the rules (REST API, plain http; empty URL: `http://<broker>:8123`). See
-[Home Assistant](home-assistant.md).
+pages and the rules (REST API, plain http; empty URL: `http://<broker>:8123`). The Plex
+user for `{plex_user}` in entity ids. See [Home Assistant](home-assistant.md).
 
 ![Home Assistant](../Pictures/web-ui/home-assistant.png)