API-created events (and any event whose `color_theme` is NULL) had two
visible bugs in the admin edit page (#550 follow-up — PR #552 fixed the
v1 POST write path, this fixes the read/save path):
1. The theme picker initialised to the hardcoded `GALLERY_THEME_PRESETS
.default.config` ("Classic Grid", green) — which had nothing to do
with the admin's actual branding palette, while the gallery itself
was rendering with the branding theme. Confusing visual mismatch.
2. Saving the event for ANY reason (changing the date, password, etc.)
wrote `color_theme = 'default'` back to the row because the save
handler always emitted the picker's initial preset name. That
silently replaced "inherit from branding" with the literal Classic
Grid preset, so the gallery's visuals jumped.
Two fixes, both in EventDetailsPage:
- Add a `themeChanged` flag, defaulted false. Flip in the picker's
onChange / onPresetChange / onSyncFromBranding callbacks. The save
handler now only writes `updateData.color_theme` when the flag is
true, so saving without touching the picker preserves NULL.
- When `event.color_theme` is null and `publicSettings.theme_config`
(the site branding) is available, initialise `currentTheme` from
branding instead of the Classic Grid preset, with currentPresetName
set to 'custom' (since inherited branding isn't a named preset).
Falls back to the Classic Grid preset only when no branding theme
exists either.
Combined effect: opening an API-created event shows the same palette
the gallery uses, and saving without changing the theme preserves the
inheritance. Existing events with a stored color_theme are unaffected
(themeChanged stays false → no write, just like before for the
common no-change-to-theme save).