Fix bad hourly start timestamp on chart data

[benvinegar]

Fixes #60

Originally this tried to do some clever tz math inside Analytics Engine, but it doesn't work. Instead just explicitly calculate the utc start time, treat everything as utc, then convert back to local during UI rendering.

Also removes formatDateString because I think it was also doing some implicit timezone conversion in there somewhere. I found dayjs to be more predictable.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.06%. Comparing base (26b539d) to head (72a46cd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
- Coverage   79.28%   79.06%   -0.23%     
==========================================
  Files          18       18              
  Lines        2269     2245      -24     
  Branches      148      145       -3     
==========================================
- Hits         1799     1775      -24     
  Misses        465      465              
  Partials        5        5              

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