Maintain consistent priority when using subject tags

[sjd210]

Pages are augmented with a subjectId when their tags are processed, however previously in cases of multiple subject tags (and for which there was no subject in the url), the ID would be whichever is the first in the doc's tag list. The order of this tag list should not matter (and indeed does not appear to be preserved when indexed) - instead we should use the subject priority that is already used for pageContext: [physics -> maths -> chemistry -> biology]

This was leading to mismatched page and title icon themes, although this could have also been avoided with a consistent order of pageContext?.subject ?? doc.subjectId (rather than the other way around), or by not re-stating the subject unnecessarily in the TitleAndBreadcrumb. To help avoid anything similar coming up again, these have both also been addressed.

[codecov]

Codecov Report

❌ Patch coverage is 30.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.20%. Comparing base (63b48ea) to head (4c99efc).

Files with missing lines	Patch %	Lines
src/app/services/tagsPhy.ts	14.28%	6 Missing ⚠️
src/app/components/pages/Concept.tsx	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #2272    +/-   ##
========================================
  Coverage   43.20%   43.20%            
========================================
  Files         601      601            
  Lines       25757    25757            
  Branches     8595     7657   -938     
========================================
  Hits        11128    11128            
- Misses      14572    14580     +8     
+ Partials       57       49     -8     

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