Auto-detect platform by default; use DOCKER_DEFAULT_PLATFORM for cross-compile targets

Remove hardcoded platform from compose file so plain 'make up' (or
'docker compose up --build') always builds natively for the host.
Explicit targets (up-arm64, up-armv7, etc.) set DOCKER_DEFAULT_PLATFORM.

infra/Makefile

@@ -1,32 +1,33 @@
 # HIY — docker compose helpers
 # Usage: make <target>
 #
-# Supported platforms
-#   up-amd64   — x86-64 (Mac Intel, Linux desktop, CI)
-#   up-arm64   — ARM 64-bit (Mac M1/M2/M3, Pi 4 / Pi 5 running 64-bit OS)
-#   up-armv7   — ARM 32-bit v7 (Pi 2 / Pi 3 / Pi 4 running 32-bit OS)
-#   up-armv6   — ARM 32-bit v6 (Pi Zero, Pi 1)
+# Default (make up) — auto-detects the host platform.
 #
-# The default target builds for linux/amd64 (same as 'up-amd64').
+# Explicit cross-compile targets:
+#   up-amd64 / up-x64 / up-win  — linux/amd64  (Mac Intel, Windows, Linux x86)
+#   up-arm64                     — linux/arm64  (Mac M1/M2/M3, Pi 4/5 64-bit OS)
+#   up-armv7                     — linux/arm/v7 (Pi 2/3/4 32-bit OS)
+#   up-armv6                     — linux/arm/v6 (Pi Zero, Pi 1)
 
 COMPOSE = docker compose
 UP      = $(COMPOSE) up --build
 
 .PHONY: up up-amd64 up-x64 up-win up-arm64 up-armv7 up-armv6 down logs
 
-up: up-amd64
+up:
+	$(UP)
 
 up-amd64 up-x64 up-win:
-	PLATFORM=linux/amd64   $(UP)
+	DOCKER_DEFAULT_PLATFORM=linux/amd64   $(UP)
 
 up-arm64:
-	PLATFORM=linux/arm64   $(UP)
+	DOCKER_DEFAULT_PLATFORM=linux/arm64   $(UP)
 
 up-armv7:
-	PLATFORM=linux/arm/v7  $(UP)
+	DOCKER_DEFAULT_PLATFORM=linux/arm/v7  $(UP)
 
 up-armv6:
-	PLATFORM=linux/arm/v6  $(UP)
+	DOCKER_DEFAULT_PLATFORM=linux/arm/v6  $(UP)
 
 down:
 	$(COMPOSE) down

infra/docker-compose.yml

@@ -18,7 +18,6 @@ services:
 
   # ── Control plane ─────────────────────────────────────────────────────────
   server:
-    platform: ${PLATFORM:-linux/amd64}
     build:
       context: ..
       dockerfile: infra/Dockerfile.server