The starter cards now carry the same circuit thumbnails the examples
gallery uses (ExampleThumbnail → /examples-thumbs/<id>.webp with the
CircuitPreview SVG fallback): boards with a Blink example reuse its thumb,
the rest get freshly captured starter-<kind>.webp shots in the same folder
(XIAO S3/C3, Netduino 2, Pi 4/5; plus the missing xiao-esp32c6-blink and
xiao-rp2040-blink example thumbs).
The ESP32 section is now one card per chip generation with the Seeed XIAO
variant preferred (DevKit V1 / XIAO S3 / XIAO C3 / XIAO C6), and XIAO
RP2040 joins the Raspberry Pi section — replacing the previous
every-variant list and the separate XIAO section.
Fixes: a pristine /editor visit clears the hardcoded Uno starter BEFORE
showing the dialog (empty canvas behind; cancel now leaves a blank
workspace), and clearing the last board re-points the editor at a
nonexistent group so Monaco no longer keeps showing the deleted board's
sketch (removeBoard only re-points when another board remains).
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