Add export and import of settings, bitmaps, pages and fonts

Export: everything in one JSON file (password and token only when ticked),
or a single page (with the bitmaps and fonts it uses) or bitmap. Import:
choose the items and the parts of the settings; items are added under a
free name unless an existing name is given, pages follow renamed bitmaps
and fonts. New endpoints: GET /api/font, GET /api/settings?secrets=1.

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

README.md

@@ -100,7 +100,7 @@ view of the display: it shows what the dots show right now, updated every 1.5 s.
 | **Draw / Live** | Draw on an 84 x 19 canvas (click or drag), or load an image: it is converted in the browser (threshold, invert, dithering, fit / fill / stretch). "Show on display now" sends it to the display right away (*live mode*); with "live while drawing" every change appears on the dots. Save it as a bitmap with a name. |
 | **Settings** | Schedule on/off, when the display is on, the late-evening quarter-hour mode, when it switches off; the "No WiFi" screen; the flip time, the pause after each pulse, pulses per dot and the refresh effect (instant, sparkle, wipe, rain, from the middle out, diagonal, spiral, flip all then reveal, push, random; optionally only for small changes like the clock's minutes); test patterns; the status LED. |
 | **Home Assistant** | 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 (REST API, plain http; empty URL: `http://<broker>:8123`). |
-| **System** | Firmware status, uptime, WiFi, storage; restart; start the recovery app. |
+| **System** | Firmware status, uptime, WiFi, storage; restart; start the recovery app. Backup: export everything (settings, your bitmaps, pages and fonts; the MQTT password and HA token only when ticked) to one JSON file, and import a file: choose which items and which parts of the settings. Imported items are added (a taken name gets a number) unless you give them an existing name. Pages and bitmaps also have their own Export button. |
 
 **Live mode** shows the image at once, also at night or before the time is known. It
 ends with "Back to normal", or 30 minutes after the last image. The Home Assistant Power