Add native Setline account sync

[sarthakagrawal927]

Tracks #22

What changed

• add exact-callback Google OAuth handoff and Better Auth bearer sessions
• keep the session token in Keychain and add native sign-in, sync, sign-out, and account deletion controls
• add revision-checked native whole-state sync with explicit device/account conflict choices
• preserve local-first workouts and freeze cloud writes when a conflict is deferred
• add the unapplied D1 migration and quiet simulator design evidence

Validation

• pnpm run check — 59 tests passed
• cd ios && ./scripts/check.sh — 13 tests passed; Release simulator build succeeded
• openspec validate add-native-ios-app --strict — passed
• personal-team archive and codesign verification — passed

Production gate

The D1 migration is committed but has not been applied, and the Worker has not been deployed. Live native account connection remains disabled until that callback/API contract is reviewed and explicitly approved.