Surveys: honor appearance.position (currently bottom-only)

Problem

posthog-react-native 4.43.13 (PostHog/posthog-js#3498) added support for honoring appearance.position on all 9 SurveyPosition values (top_left, top_center, top_right, middle_left, middle_center, middle_right, left, center, right). Survey UI on iOS still renders bottom-only regardless of the configured position.

This means surveys created in the PostHog UI with a custom position will not visually match what users see on iOS, breaking parity with the web SDK and React Native.

Expected behavior

Honor appearance.position in the iOS survey UI, matching the web SDK semantics:

• top_* → anchored to top edge
• middle_* → vertical middle
• left / center / right (no prefix) → anchored to bottom edge

Reference

• React Native implementation: https://github.com/PostHog/posthog-js/pull/3498
• Feature parity table: https://github.com/PostHog/posthog/blob/master/frontend/src/scenes/surveys/surveyVersionRequirements.ts

Start in the Flutter survey UI entry point and trace how appearance.position is currently handled, using the linked React Native implementation and web semantics as references. Done means all nine SurveyPosition values render with the specified top, middle, or bottom anchoring and horizontal alignment on iOS.