Part of per-user profile isolation: profile rows are no longer a global
singleton, so every read/write requires the caller's userID. This also
requires scoping ListActivities, ListWorkoutKinds, UpsertActivity,
CreateWorkoutKind, GetSyncState, UpdateSyncState, and ResetAllSyncedData
to userID, plus updating all related tests in the store package.
Sync now now runs Backfill (resumes from the watermark, so a widened history
horizon is picked up automatically) then IncrementalSync then detail-fill, in
one click. "Full backfill" is gone -- in its place, "Reset all" (styled as a
destructive action, gated by a confirm dialog) wipes every synced activity
and its laps/kind assignments and rewinds the backfill watermark, so the next
sync performs a genuinely fresh pull instead of trying to patch up existing
rows with newer schema fields.
