fix(designer): normalize custom study sequence input

[mohiuddinshahrukh]

Fixes #651

Summary

• Update the schedule form immediately when sequencing changes, so the custom sequence field appears and disappears without needing a refresh.
• Normalize custom sequences by stripping whitespace and uppercasing A/B values before saving.
• Reject characters outside A/B and cover the formatter/form-data behavior with focused tests.

Tested

• dart format on the touched Designer files and new tests
• flutter test test\utils\input_formatter_test.dart test\features\design\interventions\study_schedule_form_data_test.dart
• Frontend testing: custom sequence input strips spaces, uppercases letters, rejects non-A/B input, and hides immediately when switching away from Custom.