wallpaper: engine page commits like the others, Start changes at once

The page was the only settings page without the header's buttons, and
it switched the engine from a checkbox beside the interval. Both looked
like the engine had been started when it had not:

- The minutes field saved on editingFinished, which needs Enter or a
  focus change. Clicking the checkbox takes neither, so a typed 1 was
  left at 30 and the first change was half an hour away.
- Even at the right interval, the first change waited one full
  interval, so starting the engine did nothing visible.

The interval and the two sources are now a draft in WallEngine, updated
on every keystroke, and the header carries the state ("running, next
change at hh:mm", "stopped", "unsaved changes") with Reset, Stop and
Start, which reads Apply while running. Start commits the draft, sets
the homes and changes the wallpapers straight away; Apply commits and
restarts the timer. Hearts stay undrafted, written as clicked.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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