settings: one snackbar for every page's messages

Save, apply and upload results went to the page header, one line capped
at 420px and cut with an ellipsis. They now go to a snackbar at the foot
of the settings window, wrapped in full: green fades after 6s, yellow and
red stay until dismissed or replaced, and changing page clears it.

Snack.qml is the singleton pages call (ok, wait, error); Snackbar.qml
draws it once in Settings.qml. SaveBar forwards its notice and error, so
the 24 pages using it change nothing; the header keeps only the draft's
state. The boot logo page calls Snack directly.

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

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