feat: add Live Activities support to demo app

[fadi-george]

Description

One Line Summary

Add Live Activities section to the Flutter demo app with full start/update/end lifecycle support.

Details

Motivation

The demo app was missing Live Activities support. The iOS widget extension existed but the Flutter side had no UI, no SDK initialization (setupDefault), and no REST API integration for updating/ending activities.

Scope

• Flutter demo app only — no SDK changes
• Adds Live Activities section (iOS only) with Activity ID and Order # inputs
• Four actions: Start, Update (cycles through preparing → on the way → delivered), Stop Updating, End
• REST API integration for update/end via OneSignal Live Activities API
• .env-based API key configuration with flutter_dotenv
• NSSupportsLiveActivities added to main app Info.plist (was only on widget extension)
• Updated build_ios.md to document DefaultLiveActivityAttributes widget and Info.plist requirements

Testing

Manual testing

Tested on iOS simulator (iOS 26.2):

• Start Live Activity displays lock screen banner and Dynamic Island with order status
• Update cycles through statuses (preparing → on the way → delivered) via REST API
• End dismisses the Live Activity via REST API
• Stop Updating unsubscribes from remote updates via SDK
• Buttons correctly disabled when Activity ID is empty or API key is not configured

Affected code checklist

• Notifications
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
• I have reviewed this PR myself, ensuring it meets each checklist item

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