Revise starter to use localhost port sharding.

[thruflo]

This replaces the reverted subdomain sharding with port sharding. It also keeps the changes purely scoped to the starter example for now, so no changes needed to the typescript client.

Works fine with the cookie auth:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.02%. Comparing base (1360c1a) to head (148a5e2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3228      +/-   ##
==========================================
+ Coverage   74.84%   75.02%   +0.17%     
==========================================
  Files         171      171              
  Lines        8715     8715              
  Branches      107      106       -1     
==========================================
+ Hits         6523     6538      +15     
+ Misses       2191     2176      -15     
  Partials        1        1              

Flag	Coverage Δ
elixir	75.13% <ø> (+0.15%)	⬆️
elixir-client	73.94% <ø> (-0.53%)	⬇️
packages/experimental	88.95% <ø> (+1.22%)	⬆️
packages/react-hooks	86.48% <ø> (ø)
packages/y-electric	55.12% <ø> (ø)
postgres-140000	75.08% <ø> (+0.10%)	⬆️
postgres-150000	75.20% <ø> (+0.22%)	⬆️
postgres-170000	74.97% <ø> (?)
postgres-180000	75.09% <ø> (?)
sync-service	75.25% <ø> (+0.22%)	⬆️
typescript	72.85% <ø> (+0.45%)	⬆️
unit-tests	75.02% <ø> (+0.17%)	⬆️

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