refactor(frontend): time interval units

[eatyourgreens]

Store time interval units as objects with conversion factors, symbols, and unit IDs.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 76.17%. Comparing base (c227f57) to head (5bb2d48). Report is 15 commits behind head on ui-enhancements.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## ui-enhancements #577 +/- ## ================================================ Coverage 76.17% 76.17% ================================================ Files 108 108 Lines 5600 5600 ================================================ Hits 4266 4266 Misses 1334 1334 ```

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[sonarcloud[bot]]

Quality Gate passed for 'pkpdapp'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarcloud[bot]]

Quality Gate failed for 'pkpdapp-team_pkpdapp_frontend'

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud