ci(android): create placeholder app.env so compile-check bundles assets (#8828)

The android-compile-check gate (added in #8827) failed at asset bundling —
'No file or variants found for asset: app.env' — because app.env is a declared
pubspec asset that only the release workflow decodes (from a secret). Kotlin and
Dart compiled fine; the build only choked at bundling. This gate is compile-only
and never runs the app, so touch an empty app.env to satisfy bundling without
needing the secret.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: myleshorton

.github/workflows/android-compile-check.yml

@@ -124,6 +124,12 @@ jobs:
           make pubget
           make gen
 
+      # app.env is a declared pubspec asset, normally decoded from a secret by
+      # the release workflow. This compile-only gate doesn't run the app, so an
+      # empty placeholder is enough to satisfy asset bundling without the secret.
+      - name: Create placeholder app.env
+        run: touch app.env
+
       # Builds the gomobile AAR, then compiles the debug APK (all app Kotlin).
       # A Kotlin compile error fails this step — the whole point of the gate.
       - name: Build debug APK (compiles Kotlin)