ci(android): never ship a debug-signed APK; check the signer (#5116)

Adopts the rest of family idea #5103 (distributing your own APK):

- Practice 2: build.gradle.kts no longer falls back to the debug key
  when ANDROID_KEYSTORE_PATH is unset; the release build is signed with
  the release key or left unsigned. Main no longer builds and uploads a
  debug-signed app-debug.apk, which no install could ever update.
- Practice 3: android-release runs apksigner on the built APK, prints
  the signer's DN and SHA-256 digest, and fails on a debug signer.
  An unsigned build fails the same step, since there is no
  app-release.apk to verify.
- Practice 9: debug builds offer no server update. The banner does not
  poll and the About card says updates come from Android Studio, since
  the release-signed APK cannot install over a debug-signed app.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

android/app/build.gradle.kts

@@ -72,9 +72,13 @@ android {
                 getDefaultProguardFile("proguard-android-optimize.txt"),
                 "proguard-rules.pro",
             )
+            // Signed with the release key or not at all. Falling back to the
+            // debug key made a missing secret into a published APK that no
+            // install could ever update (family idea #5103, practice 2). An
+            // unsigned build installs nowhere, so the gap shows at once.
             signingConfig =
                 if (System.getenv("ANDROID_KEYSTORE_PATH").isNullOrEmpty()) {
-                    signingConfigs.getByName("debug")
+                    null
                 } else {
                     signingConfigs.getByName("release")
                 }