Firmware 1.2.0

Firmware 1.2.0 for every supported reader. Only the LilyGo T5-2.13 (V2.3.1) has been
tested on hardware; the T5-4.7" S3, M5PaperS3 and M5Paper builds have not run on a
board yet (docs/checkpoints.md lists what to check; reports welcome).

Board	Flash over USB (at 0x0)	Update over Wi-Fi
LilyGo T5-2.13	t5-reader-1.2.0.bin	t5-reader-1.2.0-ota.bin
LilyGo T5-4.7" S3	t5-47-reader-1.2.0.bin	t5-47-reader-1.2.0-ota.bin
M5Stack M5PaperS3	m5papers3-reader-1.2.0.bin	m5papers3-reader-1.2.0-ota.bin
M5Stack M5Paper	m5paper-reader-1.2.0.bin	m5paper-reader-1.2.0-ota.bin

Install from a browser: Chrome or Edge, https://daafip.github.io/esp32-ereader/,
pick your board, Install. Leave "Erase everything" unticked to keep settings, bookmarks
and reading positions (books on the SD card are never touched).
T5-2.13: remove the SD card first. T5-4.7" / M5PaperS3: if no port shows up, hold BOOT,
press RST, release BOOT (M5PaperS3: see M5Stack's download-mode instructions).

Or with https://espressif.github.io/esptool-js/: Flash Address 0x0, the .bin
without -ota. This image erases the reader's settings (not the books).

Update over Wi-Fi (reader already running this firmware): Menu → Wi-Fi transfer,
open the page, "Update firmware", choose the -ota.bin for your board. A file for
the other chip (ESP32 vs ESP32-S3) is refused; between boards with the same chip, take care.

Diagnostic sketches for the T5-2.13 hardware checkpoints (docs/checkpoints.md):
t5-panel_tester-1.2.0.bin, t5-button_test-1.2.0.bin, t5-sd_test-1.2.0.bin,
t5-calibration-1.2.0.bin.

Books: drop an EPUB on the reader's Wi-Fi page (it typesets for the reader's screen),
or pixi run prep book.epub (--screen 4.7in for the 4.7" readers). See docs/usage.md.