[codex] add iOS mobile shell and push notifications

[haowei2000]

Summary

• expands the native iOS app with the drawer shell, Activity and Agents surfaces, approval cards/sheets, file previews, and richer chat controls
• adds iOS push routing and device registration APIs, plus gateway APNs/direct relay push delivery support
• documents the mobile app design/prototype and records the App Store release-readiness checklist

Impact

• mobile users can review approval requests, inspect agent activity, manage chat context, and receive OS push notifications once production Apple credentials/relay are configured
• gateway deployments get a new idempotent user device table and device endpoints; push delivery is fire-and-forget and leaves in-app WebSocket notifications intact

Validation

• PASS: cd server && cargo test
• PASS: xcodebuild -project apps/ios/Cheers.xcodeproj -scheme Cheers -destination 'generic/platform=iOS Simulator' build
• WARN: cd server && cargo fmt --check currently fails on existing committed formatting drift in unrelated server files; this PR did not run broad rustfmt to avoid mixing unrelated churn

Release Notes

• App Store release is still blocked on production Apple signing, APNs entitlement/provisioning, production HTTPS configuration, screenshots/metadata, privacy details, review account, and production gateway/APNs or relay deployment.