Adds the opt-in foreground check and an explicit Check now, the waiting-to-
be-delivered list on the sender side, and honest reporting when a send could
not be delivered: a closed app is a delay, not a success nobody received.
The setting is off by default and its footer states that checking reveals
app-open times to remembered devices, since that is the reason it is a
setting at all.
Records in the design doc that this shipped as one global toggle rather than
the per-contact opt-in originally specified.
Track the Affinity design master (AppStore.af) with Git LFS to keep repo
history lean, and ignore the large exported JPG screenshots (regenerated from
the source) plus macOS/editor junk.
