UIEUS-523: Show 'Please enter a valid date' instead of 'Required' for invalid periodic harvesting start date

[slaemmer]

Fixes UIEUS-523.

On the periodic harvesting form, entering an invalid start date such as 2026-10-77 showed the validation message "Required" even though the field was not empty. The Datepicker commits an unparseable entry as an empty value, so the required validator could not distinguish an invalid entry from an empty field.

Changes

• Add requiredValidDate validator that returns a localized "Please enter a valid date" for an empty/invalid value and use it on the start-date field
• Add translation key errors.enterValidDate
• Add unit tests for the validator and the form's invalid-date case

[github-actions]

Jest Unit Test Results

219 tests  +3   219 ✅ +3   2m 47s ⏱️ +12s
 39 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit 7e04816. ± Comparison against base commit de7ca35.

♻️ This comment has been updated with latest results.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud