[NU-1940] Add seconds to sample generator period editor and remove white arrows

[mslabek]

Describe your changes

Changelog entry for this PR is in #7376

2 changes:

• add seconds field to period editor for sample generator (BE config)
• get rid of white arrows caused by input type=number. added some logic in https://github.com/TouK/nussknacker/pull/7373/files#diff-8773d281000063ecd74bda2c023f3a835e0bd99504543a657f32cab1932a60f3R27-R35 to not allow non-numbers - same as input type=number.

Before

After

Checklist before merge

• Related issue ID is placed at the beginning of PR title in [brackets] (can be GH issue or Nu Jira issue)
• Code is cleaned from temporary changes and commented out lines
• Parts of the code that are not easy to understand are documented in the code
• Changes are covered by automated tests
• Showcase in dev-application.conf added to demonstrate the feature
• Documentation added or updated
• Added entry in Changelog.md describing the change from the perspective of a public distribution user
• Added MigrationGuide.md entry in the appropriate subcategory if introducing a breaking change
• Verify that PR will be squashed during merge

[github-actions]

created: #7377
⚠️ Be careful! Snapshot changes are not necessarily the cause of the error. Check the logs.

[arkadius]

LGTM

[arkadius]

Please add changelog entry