[PR Preview] Integrate iOS workflow with freighter-config (sandbox URLs) #859

Goal

Update the Phase 1 mobile preview workflows (prPreviewIos.yml and prPreviewAndroid.yml) to fetch the PR author's sandbox URLs from freighter-config at build time, replacing the staging URLs hard-coded in Phase 1.

Acceptance criteria

• iOS Simulator and Android Emulator workflows fetch freighter-config entry for the PR author at build time
• PR opened by an engineer with a freighter-config entry produces a build whose backend env points at their sandbox URLs
• PR opened by an engineer without a freighter-config entry falls back to staging and posts a PR comment explaining why
• If freighter-config is unreachable, falls back to last-known cached URLs and labels the PR preview-degraded
• Build's sticky PR comment / release notes call out which backend the preview is pointed at (sandbox vs staging fallback)

Dependencies

• Blocks on [PR Preview] Create freighter-config repo for engineer → sandbox URL map freighter#2765 (freighter-config repo creation)
• Sandbox-backed mobile previews require sshuttle on the reviewer's Mac — physical-device testing is deferred to Phase 3 (Tailscale) by design

Reference

Fullstack PR Preview Flow design doc — § Pathways → Mobile, § Dev Environments Per Engineer