fix(form): skip groups that only contain skippable fields

[syf2211]

Summary

Fixes #468 by skipping groups that only contain skippable fields at form start, while preserving intentional blocking welcome screens and navigation behavior.

Motivation

A skippable note as the only field in a group was not skipped when other groups contained interactive fields. The common workaround was adding a blank second note, which affected formatting.

Changes

• Only force notes to be non-skippable when they are the only field in the entire form (not merely the only field in their group)
• Treat notes with Next(true) as blocking so burger-style welcome screens still appear
• Skip groups where every field is skippable during Form.Init(), similar to hidden groups
• Only auto-advance past skippable fields in Group.Init() when the group has more than one field
• Improve doAllUpdates test helper to process tea.Sequence messages from Form.Init()

Tests

go test ./...

All tests pass, including new regressions:

• TestSkipSingleFieldGroup
• TestSkipSingleFieldOnlyGroup
• TestSkipNextButtonWelcomeGroup

Notes

• Consecutive skippable-only groups before an interactive group may still require multiple nextGroup transitions; only the first such group is skipped at init today. Happy to follow up if maintainers want full chaining.
• Backward navigation into a skippable-only group still shows the note, which seems preferable for welcome content.