fix(home): "Next update" preview reflects when settings actually reach the frame

The frame is asleep on whatever schedule was active at its last poll —
saving new settings here does NOT reach it until that next scheduled
sync. The preview was claiming the *new* schedule's next slot, which
was misleading: setting "at 4 AM" while the frame is on every-1-min
should preview "in ~1 min" (next existing poll), not "at 4 AM".

Now compute the next sync from the device's CURRENT saved schedule
(lastSeenAt + interval, or next saved wakeTime in tz). Falls back to
"when the frame next connects" for never-seen devices and "any moment"
for already-overdue ones.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

public/build/index.html

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