[EPIC] On-device data integrity & offline sync

Two verified holes can silently lose regulatory-relevant data: informed-consent upload failures are swallowed (and local state says "accepted" regardless), and an offline first launch skips backend initialization with no retry. Sensor-data buffering itself lives in `carp_backend`/CAMS (not re-implemented in the app — correct), so this EPIC scopes the app-side gaps and adopts the two existing field bugs (#498, #523) as children.

**Definition of done:** consent reaches CAWS or is visibly queued; the app recovers from offline starts without reinstall; #498/#523 root-caused.

Line references are against branch `chore/cams-2.x-upgrade` @ 560a181.