[Experimental] Update to eslint.config.js

[glass-ships]

eslint has migrated from the .eslintrc.json to the new "flat config", eslint.config.js style.

This PR is somewhat experimental and

• updates the new config style
• addresses a number of linting/formatting errors that were (for some reason) caught with the new version but not previously

Some frontend tests are currently failing, which is definitely an argument for not bringing this in just yet. Further investigation required.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.14%. Comparing base (84dab5f) to head (a404844).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #890   +/-   ##
=======================================
  Coverage   71.14%   71.14%           
=======================================
  Files          91       91           
  Lines        3136     3136           
=======================================
  Hits         2231     2231           
  Misses        905      905           

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