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.

Makefile

@@ -1,4 +1,7 @@
-.PHONY: build build-release frontend backend run migrate reconcile reconcile-delete tidy test testdata clean
+.PHONY: build build-release frontend backend android run migrate reconcile reconcile-delete tidy test testdata clean
+
+# Android SDK location. Override when the SDK lives elsewhere.
+ANDROID_SDK ?= $(HOME)/Android/sdk
 
 # Release version stamped into the binary (-X main.version). Override with VERSION=...
 VERSION ?= $(shell git describe --tags --always --dirty 2>/dev/null || echo dev)
@@ -25,6 +28,10 @@ backend:
 
 build: frontend backend
 
+# Android client (independent of the server build). Requires the Android SDK.
+android:
+	cd android && ANDROID_HOME=$(ANDROID_SDK) ./gradlew :app:assembleDebug
+
 # Release build: full static, stripped and (if available) UPX-compressed binary
 # with the version embedded. Produces bin/galo and a copy at bin/galo-$(VERSION).
 build-release: frontend