feat: add platform-specific base branch overrides

[piotr-iohk]

Summary

• Adds platform-aware fallback logic to determine-e2e-branch.yml so iOS and Android can resolve different base branches when no matching feature branch exists
• Uses github.repository (inherited from the caller) to detect the platform — no changes needed in bitkit-ios or bitkit-android workflows
• Currently sets iOS base to feat/home-redesign while Android stays on main, until feat/home-redesign is merged on Android

How it works

A BASE_OVERRIDES map at the top of the script controls the fallback per platform:

declare -A BASE_OVERRIDES=(
  ["ios"]="feat/home-redesign"
  ["android"]="main"
)

Set both back to main once platforms are aligned again.

Test plan

• Verify iOS E2E runs pick up feat/home-redesign as fallback (trigger a PR build on bitkit-ios)
• Verify Android E2E runs still fall back to main (trigger a PR build on bitkit-android)
• Verify explicit e2e_branch_input=main still forces main on both platforms

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