Base the ladder-fields row's initial hidden state on the first segment

Task review for #1756 found a real flash bug: _form.html.slim computed
ladder_hidden from Workout#governing_segment, but ladder_fields_controller.js
resolves its rounds/time/interval targets to the first segment in DOM
order (the one the JS was wired to in this task). Those only agree
when a workout has exactly one segment or exactly one schemed segment.

Confirmed via direct computation against live seed data: at least two
real workouts (Dallas 5, Nukes -- multiple schemed segments, so no
governing segment, but an AMRAP-shaped first segment) had
server-rendered hidden=true flip to visible the instant the
controller's connect() ran, since it reads the first segment
regardless. Fixed by sourcing ladder_hidden from `segments.first`
instead, matching what the JS actually targets.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: Yanchek99

app/views/workouts/_form.html.slim

@@ -1,5 +1,5 @@
 - cancel_path = @workout.persisted? ? workout_path(@workout) : workouts_path
-- ladder_segment = @workout.governing_segment
+- ladder_segment = @workout.segments.first
 - ladder_hidden = ladder_segment.blank? || ladder_segment.time_seconds.blank? || ladder_segment.rounds.present? || ladder_segment.interval_scheme.present?
 
 = simple_form_for @workout, wrapper: :vertical_form, html: { class: 'workout-form', data: { controller: 'ladder-fields' } } do |f|