Build Encrypted Field Display Widget

Use a `StatefulWidget` with `WidgetsBindingObserver` to listen for `AppLifecycleState.paused` and call `setState(() => _revealState = RevealState.masked)`. Store `_decryptedValue` as a nullable `String?` and set it to `null` on re-mask. Use a `Semantics` widget with `onTap` override to ensure VoiceOver/TalkBack exposes the tap affordance. The warning dialog must use `showDialog()` with `barrierDismissible: false` so the user must make an explicit choice.

Guard the `recordReceipt` call with a session-level flag (stored in the provider, not the widget) so it fires only once per app session per field identifier. Use `AnimatedSwitcher` for the masked-to-revealed transition to avoid layout jank. The `PreviouslyViewedBadge` should be a small, accessible chip that does not interfere with the tap target of the main widget.

Widget tests (flutter_test): (1) verify masked placeholder is shown on initial render; (2) tap widget — verify warning dialog appears before any decryption; (3) cancel dialog — verify field remains masked and no decrypt/receipt call made; (4) confirm dialog — mock decryption success and verify revealed text is shown; (5) mock decryption failure — verify error message shown, no partial data; (6) verify `recordReceipt` called exactly once on first reveal, not on second; (7) verify `watchReceiptStatus` returning `read` shows 'Previously viewed' badge in masked state; (8) simulate `AppLifecycleState.paused` — verify field re-masks. Accessibility: verify Semantics labels in both masked and revealed states. Security: verify decrypted value is not present in widget state after re-mask. Target ≥90% line coverage.