[ui] fix: send collection intervals as numbers

[cananoo]

Description

Fixes #11815

The collection source form keeps input values as strings, so a numeric update interval was serialized as a JSON string and rejected by the Go API. This change:

• converts the interval at the API boundary so it is sent as a JSON number
• constrains the field to positive integer minutes, matching the backend contract
• adds a Playwright regression test that inspects the posted JSON body
• documents the collection source endpoints and interval unit

Notes for Reviewers

The API behavior and default interval are unchanged. Empty intervals remain omitted so the server continues to apply its existing default.

Signed commits

• Yes, I signed my commits.
• Documentation updated (docs/content/) for user-facing changes, or not applicable

Validation

• npx eslint e2e/collections.spec.js src/utils/api.js src/pages/CollectionDetails.jsx (0 errors; existing warnings only)
• npm run lint:inline-styles
• npm run build
• npx playwright test e2e/collections.spec.js --project=chromium --list
• Full Playwright execution was not available in this Windows environment because the repository's configured UI test server uses Unix paths and requires the generated Go test assets.