fix: make Android signing config conditional on ANDROID_KEYSTORE_PATH (#440)

## Summary

- `BuildAndroidRelease` in `ci/main.go` intentionally builds the AAB without setting up the keystore — the unsigned AAB is later stamped with `StampAndroidVersionCode` and re-signed by `SignAndroidBundle` via jarsigner.
- The old `signingConfigs.create("release")` block in `android/app/build.gradle.kts` called `error("ANDROID_KEYSTORE_PATH is not set")` at Gradle _configuration_ time, which fired even when the keystore wasn't needed for the build step.
- Fix: guard the `signingConfigs` block and the `signingConfig` assignment in the release build type behind a null-check on `ANDROID_KEYSTORE_PATH`. When the env var is absent (unsigned build path), Gradle skips the signing config entirely; when it is present (e.g. `BuildAndroidApk` via `setupKeystore`), the config is created and applied as before.

## Test plan

- Trigger `deploy.yml` via `workflow_dispatch` and verify the `Build & Deploy to Play Store` job no longer fails at step 4 with "ANDROID_KEYSTORE_PATH is not set"
- Verify `BuildAndroidApk` (which calls `setupKeystore`) still produces a correctly signed APK

Closes #439

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/440

android/app/build.gradle.kts

@@ -22,13 +22,17 @@ android {
         }
     }
 
-    signingConfigs {
-        create("release") {
-            keyAlias = "upload"
-            val pass = System.getenv("ANDROID_KEYSTORE_PASSWORD")
-            storePassword = pass
-            keyPassword = pass
-            storeFile = file(System.getenv("ANDROID_KEYSTORE_PATH") ?: error("ANDROID_KEYSTORE_PATH is not set"))
+    val ksPath: String? = System.getenv("ANDROID_KEYSTORE_PATH")
+
+    if (ksPath != null) {
+        signingConfigs {
+            create("release") {
+                keyAlias = "upload"
+                val pass = System.getenv("ANDROID_KEYSTORE_PASSWORD") ?: ""
+                storePassword = pass
+                keyPassword = pass
+                storeFile = file(ksPath)
+            }
         }
     }
 
@@ -44,7 +48,9 @@ android {
 
     buildTypes {
         release {
-            signingConfig = signingConfigs.getByName("release")
+            if (ksPath != null) {
+                signingConfig = signingConfigs.getByName("release")
+            }
             isMinifyEnabled = false
             isShrinkResources = false
             ndk {