feat: shorten weekly period display labels

[eirikhaugstulen]

Summary

Shortens weekly fixed-period name and displayName values to contain only the week title, e.g. Week 1, while preserving startDate and endDate as structured fields.

Why

Weekly labels currently include the date range, e.g. Week 1 - 2025-12-29 - 2026-01-04. Consumers that need a shorter UI label have to parse this localized display string, which is fragile. Since the period object already exposes startDate and endDate, callers can compose richer labels themselves.

This also makes weekly period display names more consistent with monthly periods, where the display name is the period title and dates remain separate structured fields.

Testing

• CI=true yarn test --runInBand --watchAll=false --watchman=false src/period-calculation/generate-fixed-periods src/period-calculation/create-fixed-period-from-period-id src/period-calculation/get-adjacent-fixed-periods
• yarn cucumber
• yarn lint
• yarn build:types
• git diff --check

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.84%. Comparing base (d202f08) to head (b9b79ce).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #93      +/-   ##
==========================================
- Coverage   90.87%   90.84%   -0.03%     
==========================================
  Files          57       57              
  Lines        1249     1245       -4     
  Branches      352      352              
==========================================
- Hits         1135     1131       -4     
  Misses        112      112              
  Partials        2        2              

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
48.9% Duplication on New Code

See analysis details on SonarQube Cloud