Fix offline support for Earn tab - prevent crashes and add caching

[Copilot]

1. What triggered this change? (PBI link, Email Subject, conversation + reason, etc)

Issue #1308 - Earn tab (Quiz page) crashed when offline and showed error dialogs on every "Next" button press during quiz submission.

2. What was changed?

QuizViewModel - Implemented caching with offline support

• Migrated from ObservableRangeCollection to AdvancedObservableCollection with IFileCacheService
• Added OnQuizzesError() callback distinguishing HttpRequestException for network-specific messaging
• Added connectivity check in OpenQuiz() before navigation
• Added QuizItemViewModel.IsEqual() for efficient cache comparison

QuizDetailsViewModel - Added network checks at critical points

• Connectivity check in Initialise() with navigation fallback
• Silent skip in SubmitAnswer() when offline (prevents error spam on "Next")
• Connectivity check in SubmitResponses() before final submission
• Null checks for API responses

QuizPage.xaml - Updated binding

- ItemsSource="{Binding Quizzes}"
+ ItemsSource="{Binding Quizzes.Collection}"

QuizPage.xaml.cs - Added resource cleanup

• OnDisappearing() to dispose timer

QuizService - Improved error propagation

• Rethrow HttpRequestException to allow ViewModel-level handling

Pattern: Follows existing RedeemViewModel and ProfileViewModelBase offline patterns (cache-first loading, network-specific errors, silent failures with cached fallback).

3. Did you do pair or mob programming?

No

Original prompt

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This section details on the original issue you should resolve

<issue_title>🐛 Offline Support | Earn Tab [JK]</issue_title> <issue_description>Issues to fix on Mobile app:

• [ ] Prevent crash when going on earn tab
• [ ] Cache earn tab
• [ ] Gracefully handle when attempting to start a quiz
• [ ] Don't throw an error on each "Next" press in quiz when offline

Look at other pages how offline is supported.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes SSWConsulting/SSW.Rewards.Mobile#1308

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[jernejk]

@zacharykeeping, needs more testing but looks ok.