Firmware 2.0.0

Firmware 2.0.0 for every supported reader. Tested on hardware: the LilyGo T5-2.13
(V2.3.1) and the home-built FireBeetle 7.5" without a card; the FireBeetle's card
build and 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-2.0.0.bin	t5-reader-2.0.0-ota.bin
LilyGo T5-4.7" S3	t5-47-reader-2.0.0.bin	t5-47-reader-2.0.0-ota.bin
M5Stack M5PaperS3	m5papers3-reader-2.0.0.bin	m5papers3-reader-2.0.0-ota.bin
M5Stack M5Paper	m5paper-reader-2.0.0.bin	m5paper-reader-2.0.0-ota.bin
FireBeetle 2 ESP32-C6 + 7.5"	firebeetle-75-reader-2.0.0.bin	firebeetle-75-reader-2.0.0-ota.bin
the same without a microSD card	firebeetle-75-flash-reader-2.0.0.bin	over USB only

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 / FireBeetle: 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
another chip (ESP32, ESP32-S3, ESP32-C6) is refused, and so is another board's file.

Diagnostic sketches for the T5-2.13 hardware checkpoints (docs/checkpoints.md):
t5-panel_tester-2.0.0.bin, t5-button_test-2.0.0.bin, t5-sd_test-2.0.0.bin,
t5-calibration-2.0.0.bin; for the FireBeetle 7.5": firebeetle-75-test-2.0.0.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, 7.5in or
7.5in-landscape for the FireBeetle). Since 2.0.0 books are compressed (.t5b
version 2, about 5 times smaller): they need firmware 2.0.0 or later, and older books
still open (prep --uncompressed makes books for 1.x firmware). A FireBeetle without a
card keeps its books in its flash, room for about 170 pages: of a bigger book the page
sends the part you choose. See docs/usage.md.