Fix: Add optional content visual indicator (fix #259)

[joe-replin]

Fix

• Fixes Completion calculations can return NaN when page content is optional #259
• Fixed division by zero errors in completionCalculations.js when page contains only optional content - now returns 0 instead of NaN
• Fixed division by zero in PageLevelProgressIndicatorView.js when components have no children

New

• Added visual "Optional" label below progress indicators when isOptional is true.
• Added "Optional." prefix to aria-label for screen reader announcements
• Implemented conditional wrapper rendering when label exists.

Testing

1. Set "_isOptional": true on page content (manually or via diagnostic extension)
2. Navigate to menu and verify:
3. Progress indicator shows "Optional" label below the progress bar
4. Empty progress bar displays correctly (0% instead of appearing full due to NaN)
5. Navigate within the optional page and check the navigation bar progress indicator
6. Test with screen reader - verify it announces "Optional" before progress information
7. Test non-optional content - verify no extra wrapper elements or labels appear
8. Inspect DOM - confirm pagelevelprogress__indicator-group only exists when content is optional

[cahirodoherty-learningpool]

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