Append a "Custom" entry to the theme dropdown, right after "Auto (dynamic)"
(theme::CUSTOM_SLUG/custom_index, mirroring the AUTO_SLUG pattern). New
crate::custom_theme persists the CustomThemeBase to
<data_dir>/qbz/custom_theme.json (load/save mirror ui_prefs) and derives it
via qbz_theme::theme_from_base.
Startup: when the persisted theme slug is "custom", seed the editor swatches
from custom_theme.json and apply the derived palette (load_or_seed seeds the
OLED default the first time). The dropdown "theme" arm gains a Custom branch
with the same live-apply path. AppearanceState grows theme-is-custom, the 11
custom-* swatch colors, custom-is-dark, custom-open-token and the
custom-set-token / custom-seed-from-current / custom-toggle-dark callbacks
(handlers wired in a later commit).
cc
vicrodh
c6a7527bbb