Add nightly iOS/Android build workflow

Add a scheduled workflow (2:10 AM UTC, after the nightly i18n job) that
builds both platforms via reusable workflow_call triggers:
- Android: testflight-android profile, producing an Internal release
- iOS: testflight profile, then assigns the build to the "QA Team"
  TestFlight group via fastlane (distribute_only)

Adds workflow_call triggers + an assignTestFlightGroup input to the iOS
workflow. Assigning to a TestFlight group requires App Store Connect API
key secrets (ASC_KEY_ID, ASC_ISSUER_ID, ASC_KEY_P8_BASE64), which still
need to be added to the repo.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/build-submit-android.yml

@@ -10,6 +10,12 @@ on:
         options:
           - testflight-android
           - production
+  workflow_call:
+    inputs:
+      profile:
+        type: string
+        description: Build profile to use
+        required: true
 
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