Prevent stale content components

[jacbn]

Fixes an issue spotted on the SPC tests owing to identical DOM structures between sections erroneously preserving question state.

Description in the code explains this well:

Each <IsaacContent/> is assumed to be independent, not sharing state with any other. However, React will reuse components if they are the same type, have the same key (or undefined), and exist in the same place in the DOM. If two components A and B meet these criteria, if you switch from component A to component B, any e.g. useStates in B will not initialise as expected, but will retain stale data from A.

This is a problem for any structure where one of several <IsaacContent/>s are displayed, e.g. quiz sections, tabs, ... .

To avoid this, we set the key of each to its content ID.

More info here: https://react.dev/learn/preserving-and-resetting-state#same-component-at-the-same-position-preserves-state.

[codecov]

Codecov Report

Attention: Patch coverage is 15.00000% with 17 lines in your changes missing coverage. Please review.

Project coverage is 40.60%. Comparing base (4afc7c1) to head (313dc8f).
Report is 25 commits behind head on redesign-2024.

Files with missing lines	Patch %	Lines
src/app/components/content/IsaacContent.tsx	15.00%	17 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           redesign-2024    #1477    +/-   ##
===============================================
  Coverage          40.60%   40.60%            
===============================================
  Files                495      495            
  Lines              22087    22088     +1     
  Branches            6531     7323   +792     
===============================================
+ Hits                8968     8969     +1     
+ Misses             13082    12494   -588     
- Partials              37      625   +588     

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