Restore web identity photo previews (#69)

docs/ux/photo-preview.md

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+# Identity photo preview correction
+
+Task [#69](https://git.sendico.io/tech/sxchnge/issues/69), branch `kyc-69`.
+This frontend change builds on the [identity verification journey](kyc.md)
+and is independent of the birthday controls in task #67.
+
+## Cause and correction
+
+A synthetic valid PNG reproduced this Flutter 3.47.7 browser error:
+
+```text
+TypeError: Instance of '_UnmodifiableUint8ListView': type
+'_UnmodifiableUint8ListView' is not a subtype of type 'NativeUint8List'
+```
+
+The stack reached `Uint8ListToJSUint8Array.toJS` from CanvasKit's browser
+image decoder through `MemoryImage` and `ResizeImage`. File selection had
+successfully decoded the original typed bytes. The `Photo` constructor then
+copied them into an unmodifiable view, which was passed directly into the
+preview's web decoder. Native decoding accepted that view, hiding the bug in
+native-only tests. The generic image error builder presented the failure as a
+damaged document even though the selected bytes were valid.
+
+`PhotoPreview` now creates a native `Uint8List` copy for `MemoryImage` once per
+selected photo. The shared domain model stays immutable, and validation,
+providers, services and transport contracts remain unchanged. The preview still
+requests a decode width of at most 1000 pixels and evicts its decoded cache entry
+on replacement or disposal. The extra encoded-byte copy belongs to the preview
+and is released with its provider/cache references; no secure-erasure guarantee
+is made.
+
+An actual preview failure now says that the preview could not be displayed and
+suggests selecting the photo again. The English/Russian fallback keeps at least
+the preview's usual height, can grow for long text, and has a separate live-region
+semantic node. Existing replacement/removal controls stay available and selected
+input is retained. This is distinct from validation's corrupt-file message.
+
+## UX and measurement review
+
+The outcome remains selecting readable ID and selfie photos, checking both on
+review, and submitting for confirmed service acceptance. Existing KYC journey
+attempts, retries, failure, submission and abandonment signals are reused. No
+control-level events, image bytes, filenames, form values or raw exceptions are
+added to telemetry. A selection success measures validation; it does not establish
+that pixels were painted. Preview-only rendering failures are not separately
+counted by existing telemetry. Server review/approval measurement and native
+screen-reader QA retain the limitations documented in the original journey.
+
+Regression tests use synthetic JPEG/PNG content only. They start and await real
+image decoding outside the widget test's fake async zone, inspect the actual
+rendered images, and surface stream errors so an error fallback cannot conceal a
+failure. Checks cover portrait/landscape decoding, model immutability, bounded
+decode width, stable provider reuse, replacement/removal cache eviction, both
+review slots, returning to review, localized fallback/replacement, a 320px view
+at 1.6 text scale, and error announcements isolated from surrounding instructions.
+The `image` package is already a KYC runtime dependency; its direct app test
+dependency is used only to create synthetic fixtures in memory.
+
+## Validation and delivery
+
+The original browser regression failed before the fix while its native equivalent
+passed. The complete task diff was reviewed against frontend architecture, UX,
+measurement, lifecycle and privacy requirements. Final validation passed:
+
+- In `frontend/app`, `dart format --output=none --set-exit-if-changed
+  lib/kyc/preview.dart test/preview_test.dart`: two files, no changes.
+- In `frontend/app`, `flutter analyze --no-pub`: no issues.
+- In `frontend/app`, `flutter test --platform chrome test/preview_test.dart
+  --reporter expanded --no-pub --verbose`: all six regression tests passed.
+- At the repository root, `make test-frontend`: all 228 tests passed across
+  the shared packages, theme example and app, including the six native preview
+  regressions and existing KYC journey/analytics tests.
+- In `frontend/app`, `flutter build web --no-pub`: release build and Wasm
+  compatibility dry run succeeded.
+
+Browser regression coverage uses Chrome; Safari and native assistive-technology
+testing remain release checks. Generated localization and build code stays
+ignored; the tracked app lockfile changes only `image` from transitive to direct
+test dependency. The separate birthday worktree was preserved.