[Bug]: Dropdown popup renders below the helper text instead of over it

[soulcramer]

Outcome

The Dropdown popup overlays the helper text field rather than rendering beneath it.

Problem

In Dropdown, the dropdownContent popup appears below the helper field instead of overlaying it, causing a broken visual layout. Confirmed in #lbc-spark-android.

Context

Current behaviour: the popup is positioned after the helper text in the layout stack, so it renders below it. This cannot be fixed until the FormField component lands on Android — this issue is blocked on that dependency.
Source file: spark/src/main/kotlin/com/adevinta/spark/components/textfields/Dropdown.kt

Scope

• Reposition the dropdownContent popup so it overlays the helper text once FormField is available on Android.

Acceptance criteria

• The Dropdown popup appears over the helper text, not below it.
• A UI test confirms the popup z-order relative to the helper field.

Evidence

• Slack: https://adevinta.enterprise.slack.com/archives/C04QQEU3Y2H/p1777451074946939 — David C., 2026-04-29
• spark/src/main/kotlin/com/adevinta/spark/components/textfields/Dropdown.kt — popup placement logic to be updated once FormField is available