Fix: Prevent NaN progress calculations (fixes #259)

[joe-replin]

Fix

• Fixes Completion calculations can return NaN when page content is optional #259
• Prevent division-by-zero that produced NaN for page-level progress when a page contains only optional content.
• Page-level progress now consistently returns 0 for fully-optional pages/components instead of NaN, preventing invalid CSS such as --adapt-pagelevelprogress-percentage: NaN%; which previously made the indicator render incorrectly.

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

[joe-replin]

Thanks, @swashbuck.

[swashbuck]

👍