feat: add ClearAllNotifications and simplify demo build guide

[fadi-george]

Description

One Line Summary

Addresses: #133

Add ClearAllNotifications to the SDK interface and wire it up in the demo app, plus simplify the demo build guide to reference the shared sdk-shared docs.

Details

Motivation

The demo app was missing a way to clear all delivered notifications. Other wrapper SDKs already expose this. The build guide was also duplicating content that now lives in the shared sdk-shared/demo/build.md, so it was trimmed to only contain .NET MAUI-specific details.

Scope

SDK interface gains ClearAllNotifications() on INotificationsManager, implemented on both Android and iOS. The demo app gets a new "CLEAR ALL" button in the Send Push Notification section. The build guide is restructured to reference the shared guide and only list DotNet-specific overrides.

Changes

SDK (public API)

• INotificationsManager gains void ClearAllNotifications()
• Android: delegates to OneSignalNative.Notifications.ClearAllNotifications()
• iOS: delegates to OneSignalNative.Notifications.ClearAll()

Demo app

• OneSignalRepository.ClearAllNotifications() wraps the SDK call
• AppViewModel.ClearAllNotifications() command with log message
• SendPushSection.xaml adds a CLEAR ALL button (AutomationId="clear_all_notifications_button")
• SendPushSection.xaml.cs wires the click handler

Build guide

• examples/build.md now references the shared sdk-shared/demo/build.md and only contains .NET MAUI-specific overrides (reduced from ~1095 to ~339 lines)

Testing

Manual testing

Built and ran the demo app on Android emulator. Verified the CLEAR ALL button calls through to the SDK and clears delivered notifications from the tray.

Android

iOS

Affected code checklist

• Notifications
  • Display
• 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

[nan-li]

Public API should be OneSignal.Notifications.clearAll() to match API on all other platforms

[fadi-george]

Public API should be OneSignal.Notifications.clearAll() to match API on all other platforms

Done, for C# were using pascal case