fix: use default camera autofocus for QR scanning

[hermes-alby]

Summary

Remove the autofocus prop entirely from the shared QR scanner, using Expo's defaults on every platform as requested in review. The production diff is a one-line deletion; no platform conditional, lens/zoom override, new dependency, or UI change.

Keep this draft pending iPhone 15 Pro and Android device tests. This removes a one-shot-focus override on iOS, but is not a verified macro/lens-switching fix.

Platform behavior

• Expo Camera 57.0.3 defaults an omitted prop to off.
• iOS: off maps to continuous autofocus; on focuses once and locks. Expo's maintainer recommends the default for moving QR scans.
• Android: Expo's native default is already OFF; the prop setter only changes it when needed. This leaves CameraX's default focus handling in place. Source inspection of the pinned CameraX 1.6.0 confirms preview defaults to continuous-picture autofocus where supported (State3AControl.kt). cancelFocusAndMetering() restores continuous AF rather than disabling it. Sources: Expo view, CameraX camera2 source archive, CameraX core source archive.
• Web: this also adopts Expo's default; its off mapping requests manual focus where supported, unlike iOS. It does not preserve the previous web focus override.

Other-wallet research

• Blink: uses Camera Kit, whose native implementation prefers triple → dual-wide → dual → physical wide cameras.
• BlueWallet: uses a Camera Kit fork with motion-triggered focus reset and adjustable zoom, with the same virtual-camera preference.
• eNuts: uses Expo Camera without an autofocus override, as this PR now does.

These are implementation comparisons, not device-test results. Our pinned Expo version selects lenses by localized names, and virtual cameras need device-derived zoom normalization. No brittle lens override is added. The upstream reporter still experienced close-range blur after trying the focus setting; a physical lens's minimum focusing distance remains a separate issue.

Verification on the updated code

• RED: all three platform omission assertions failed before removing the prop; QR callback forwarding still passed.
• yarn test --runInBand — 28 passed.
• yarn test --preset jest-expo/android --runInBand --testMatch '**/__tests__/**/*.[jt]s?(x)' — the same 28 tests passed under the Android preset. The explicit glob matches this repo's existing test filenames.
• yarn lint:js, yarn tsc:compile, yarn format, git diff --check — passed.
• CI=1 yarn expo export --platform android --output-dir /tmp/alby-go-default-focus-android — passed.
• CI=1 yarn expo export --platform ios --output-dir /tmp/alby-go-default-focus-ios — passed.

Exports are JavaScript/Hermes bundles, not native APK/Xcode builds. Unit tests check the omitted-prop contract and QR callback, not optics. No Android SDK/device tooling or physical iPhone is available here. The previous fork CI Android build was blocked before compilation by missing Expo authentication; current-head CI must be checked separately.

Device checks before marking ready

• Cold native launch on iPhone 15 Pro and Android, rather than Fast Refresh. Open the scanner pointing far away, then aim at a QR and move near/far; confirm refocusing and successful scans.
• Repeat after reopening the scanner and backgrounding/foregrounding the app; record device model and OS version.
• Test small/dense QRs at normal and close distances, including dim light. Compare the iPhone with Blink using the same QR and distance.
• Smoke-test an older iPhone; check web focus behavior if that target is supported.
• If close-range blur persists, investigate capability-aware multi-camera selection separately; do not claim this patch fixes macro limitations.

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