[AAv2] Integrate on-device APIs (#9564)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/ageAssurance/const.ts

@@ -4,12 +4,29 @@ import {
 } from '@atproto/api'
 
 import {AgeAssuranceAccess} from '#/ageAssurance/types'
+import {ANDROID_API_LEVEL, IOS_MAJOR_VERSION, IS_ANDROID, IS_IOS} from '#/env'
 
 /**
  * Minimum age required to access the app at all.
  */
 export const MIN_ACCESS_AGE = 13
 
+/**
+ * Whether the current device can provide the native on-device age signals we
+ * use for age assurance (via `expo-age-range`). We gate on OS version because
+ * the underlying platform APIs are only available on:
+ *
+ * - iOS 26.0+ (Declared Age Range API)
+ *   https://developer.apple.com/documentation/declaredagerange/
+ * - Android 6.0 / API level 23+ (Play Age Signals API)
+ *   https://developer.android.com/google/play/age-signals/use-age-signals-api
+ *
+ * On unsupported platforms/versions (including web) this is `false`, so we skip
+ * the device flow and fall back to KWS.
+ */
+export const DEVICE_SIGNALS_SUPPORTED: boolean =
+  (IS_IOS && IOS_MAJOR_VERSION >= 26) || (IS_ANDROID && ANDROID_API_LEVEL >= 23)
+
 export const FALLBACK_REGION_CONFIG: AppBskyAgeassuranceDefs.ConfigRegion = {
   countryCode: '*',
   regionCode: undefined,