Support pairing without server notifications - #14

Summary

• Validate and save standard OpenCode pairing codes directly without a notification broker.
• Fall back automatically when notification setup is unavailable and explain that the server connected without notifications.
• Keep an explicit no-notifications option and HTTP approval.
• Add tests for direct pairing, denied push permission, an unreachable broker, and failed server validation.

Verification

• pnpm check passed.
• pnpm native:doctor passed all 18 checks.
• Published to the preview EAS Update channel for iOS and Android, runtime 0.1.4.
• Physical-device confirmation of automatic fallback is pending.