Upgrade latest Remix and enable all future flags

[benvinegar]

Upgrades to Remix v2.15 and enables remaining future flags:

• v3_routeConfig
• v3_lazyRouteDiscovery
• v3_singleFetch

This is to prepare Counterscale for migrating to React Router v7 in the future.

NOTE: This has the unfortunate side effect of causing a bunch of React Router Future Flag Warning output in tests. This is caused by remix-run/remix#10216.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 78.49%. Comparing base (d1485b1) to head (f079c52).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
app/root.tsx	0.00%	2 Missing ⚠️
app/routes.ts	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
- Coverage   78.55%   78.49%   -0.07%     
==========================================
  Files          33       34       +1     
  Lines        2486     2488       +2     
  Branches      265      266       +1     
==========================================
  Hits         1953     1953              
- Misses        525      526       +1     
- Partials        8        9       +1     

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