feat(make): add install-android-app target and default help listing

- add `help` target and make it the default goal (bare `make` lists targets)
- add `install-android-app`: build debug APK and install on the selected device
- device selection via SERIAL/$ANDROID_SERIAL; several devices list and stop
- document the target in android/README.md

Makefile

@@ -1,8 +1,19 @@
-.PHONY: build build-release frontend backend android run migrate reconcile reconcile-delete tidy test testdata clean
+.PHONY: help build build-release frontend backend android install-android-app run migrate reconcile reconcile-delete tidy test testdata clean
+
+# Default goal: print the list of available targets.
+.DEFAULT_GOAL := help
 
 # Android SDK location. Override when the SDK lives elsewhere.
 ANDROID_SDK ?= $(HOME)/Android/sdk
 
+# adb from the Android SDK's platform-tools (matches the SDK we build against).
+ADB ?= $(ANDROID_SDK)/platform-tools/adb
+
+# Device serial for `install-android-app` (falls back to $ANDROID_SERIAL). When
+# empty, the sole connected device is used; with several connected devices the
+# target lists them and stops instead of guessing. Override with SERIAL=<serial>.
+SERIAL ?= $(ANDROID_SERIAL)
+
 # Release version stamped into the binary (-X main.version). Override with VERSION=...
 VERSION ?= $(shell git describe --tags --always --dirty 2>/dev/null || echo dev)
 
@@ -14,6 +25,13 @@ PHOTOS ?= 10
 SEED ?= 0
 TIMEOUT ?= 30
 
+# Print the list of available targets.
+help:
+	@echo "Usage: make <target>"
+	@echo
+	@echo "Available targets:"
+	@grep -hE '^[a-zA-Z0-9_-]+:' $(MAKEFILE_LIST) | cut -d: -f1 | sed 's/^/  /'
+
 frontend:
 	cd frontend && npm ci && npm run build
 	rm -rf backend/static/*
@@ -32,6 +50,28 @@ build: frontend backend
 android:
 	cd android && ANDROID_HOME=$(ANDROID_SDK) ./gradlew :app:assembleDebug
 
+# Install the debug Android client on a connected device. Builds the APK via the
+# `android` target first; only devices in the `device` state are considered.
+install-android-app: android
+	@devices="$$($(ADB) devices | awk 'NR>1 && $$2 == "device" {print $$1}')"; \
+	if [ -n "$(SERIAL)" ]; then \
+		target="$(SERIAL)"; \
+	else \
+		count=$$(printf '%s\n' "$$devices" | grep -c .); \
+		if [ "$$count" -eq 0 ]; then \
+			echo "install-android-app: no connected device (see '$(ADB) devices')" >&2; \
+			exit 1; \
+		elif [ "$$count" -gt 1 ]; then \
+			echo "install-android-app: several devices connected; set SERIAL=<serial>:" >&2; \
+			printf '%s\n' "$$devices" | sed 's/^/  /' >&2; \
+			exit 1; \
+		else \
+			target="$$devices"; \
+		fi; \
+	fi; \
+	echo "Installing app-debug.apk on $$target"; \
+	$(ADB) -s "$$target" install -r android/app/build/outputs/apk/debug/app-debug.apk
+
 # 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