fix(ci): fail fast on empty keystore before 7-min cargo build

If KEYSTORE_BASE64 is unset, base64 -d writes an empty file silently,
cargo ndk then spends ~7 min compiling all ABIs, and only then does
apksigner fail. Add a size check after decode so the job fails in
seconds with a clear error message instead of wasting a full build.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/android-release.yml

@@ -101,7 +101,11 @@ jobs:
 
       # ── Build & sign with release keystore ─────────────────────────────
       - name: Decode keystore
-        run: echo "${{ secrets.KEYSTORE_BASE64 }}" | base64 -d > /tmp/solitaire-release.jks
+        run: |
+          echo "${{ secrets.KEYSTORE_BASE64 }}" | base64 -d > /tmp/solitaire-release.jks
+          size=$(wc -c < /tmp/solitaire-release.jks)
+          echo "Keystore size: ${size} bytes"
+          [ "$size" -gt 0 ] || { echo "ERROR: KEYSTORE_BASE64 secret is empty or unset"; exit 1; }
 
       - name: Build signed release APK
         env: