iOS TestFlight: use openssl base64 -d -A, stop fighting BSD base64

`base64 --decode` also failed on the Mac runner with the same
"(null): error decoding base64 input stream" — so either this macOS
version's BSD base64 doesn't honor --decode (only -D) or it chokes
on the input format. Either way, swap the three secret decodes to
`openssl base64 -d -A`:

  - openssl ships with macOS by default; no extra install
  - -d = decode (same on BSD and Linux, no platform skew)
  - -A treats the input as one continuous line (matches our
    `base64 -i x | tr -d '\n'` encoding flow)

If openssl base64 ever bites us we'll fall back to python3's
base64.b64decode, but openssl has been bulletproof for ~25 years.

.github/workflows/build-ios-testflight.yml

@@ -258,7 +258,7 @@ jobs:
           security list-keychains -d user -s "$KEYCHAIN_PATH" $(security list-keychains -d user | sed 's/"//g')
           # Import the .p12
           P12="$RUNNER_TEMP/dist.p12"
-          echo "$APPLE_DIST_CERT_P12_BASE64" | base64 --decode > "$P12"
+          echo "$APPLE_DIST_CERT_P12_BASE64" | openssl base64 -d -A > "$P12"
           security import "$P12" -k "$KEYCHAIN_PATH" -P "$APPLE_DIST_CERT_P12_PASSWORD" \
             -T /usr/bin/codesign -T /usr/bin/security
           security set-key-partition-list -S apple-tool:,apple:,codesign: -s -k "$APPLE_KEYCHAIN_PASSWORD" "$KEYCHAIN_PATH"
@@ -274,7 +274,7 @@ jobs:
           PROFILES_DIR="$HOME/Library/MobileDevice/Provisioning Profiles"
           mkdir -p "$PROFILES_DIR"
           PROFILE="$RUNNER_TEMP/Hash_Wallet.mobileprovision"
-          echo "$APPLE_PROVISIONING_PROFILE_BASE64" | base64 --decode > "$PROFILE"
+          echo "$APPLE_PROVISIONING_PROFILE_BASE64" | openssl base64 -d -A > "$PROFILE"
 
           # Parse the .mobileprovision to get its UUID + Name (CMS-decoded plist).
           PROFILE_UUID=$(security cms -D -i "$PROFILE" | plutil -extract UUID raw -o - -)
@@ -295,7 +295,7 @@ jobs:
         run: |
           set -e
           mkdir -p "$HOME/.appstoreconnect/private_keys"
-          echo "$APPLE_API_KEY_P8_BASE64" | base64 --decode > "$HOME/.appstoreconnect/private_keys/AuthKey_${APPLE_KEY_ID}.p8"
+          echo "$APPLE_API_KEY_P8_BASE64" | openssl base64 -d -A > "$HOME/.appstoreconnect/private_keys/AuthKey_${APPLE_KEY_ID}.p8"
           ls -la "$HOME/.appstoreconnect/private_keys/"
 
       # ---- Flutter init + codegen + pod install ---------------------------