feat(android): add native Android client

Kotlin + Jetpack Compose client for a self-hosted instance: manual
server setup and health check, auth with Keystore-backed tokens, album
and photo browsing with SSE live updates, upload queue in WorkManager
with HEIC-to-JPEG conversion, and Room/Coil offline cache. Add an
android Makefile target and revise project.md §17.

android/app/proguard-rules.pro

@@ -0,0 +1,16 @@
+# Keep kotlinx.serialization generated serializers.
+-keepattributes *Annotation*, InnerClasses
+-dontnote kotlinx.serialization.**
+-keepclassmembers class kotlinx.serialization.json.** {
+    *** Companion;
+}
+-keepclasseswithmembers class kotlinx.serialization.json.** {
+    kotlinx.serialization.KSerializer serializer(...);
+}
+-keep,includedescriptorclasses class com.galo.android.**$$serializer { *; }
+-keepclassmembers class com.galo.android.** {
+    *** Companion;
+}
+-keepclasseswithmembers class com.galo.android.** {
+    kotlinx.serialization.KSerializer serializer(...);
+}