fix: validate linked protection duration fields

[sirakinb]

Fixes #238.

This tightens the linked entry-field behavior between protection duration and protection end date:

• revalidates protected amount, duration, and end date before premium calculation
• keeps the linked duration/end-date fields validated when one updates the other
• applies min/max validation and helper text to the date picker
• recalculates premium after bundles load if the protected amount was already touched

Testing:

• git diff --check
• npm run lint -- --file src/components/application/application_form.tsx
• npm test -- --runInBand tests/components/application/application_form.test.tsx

[sirakinb]

Note on the failing CLA check: GitGuardian has passed, and the failing job is only CLAAssistant.

This appears to be the same repository-side CLA Assistant infrastructure problem seen on the other current Etherisc PRs: the workflow is failing inside the CLA Assistant step while trying to read the private/shared signature repository, not on the application code changes. Could a maintainer please rerun/fix the CLA Assistant token or advise how you want contributor CLA handled here?