File names may now carry '/' paths; the explorer renders each board group
as a collapsible folder tree (folders first, files in group order, CSS-var
indentation because .fe-file-item pins padding with !important). New-folder
buttons on the board header and per-folder hover actions (new file here,
new subfolder, delete with confirm when files are inside; a folder holding
every file of the group cannot be deleted - groups keep >= 1 file).
Model: names are the source of truth; only EMPTY folders are tracked
(folderGroups per group) so they survive until a file lands in them, then
materialise into the name prefix. Round-trips through .vlx (optional
folderGroups field, laxly validated), the auth stash draft, and
loadProjectState. Examples gain boards[].files so a single board can ship
a full multi-file tree (first entry = main sketch).
Backend: the arduino-cli sketch writer now creates subdirectories for
'/'-pathed names and REJECTS names that resolve outside the sketch dir
('..' traversal) - previously it crashed on the former and wrote outside
the tmp dir on the latter.
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