Move Odin renderer into gui/ with per-project Makefiles

Each sub-project (gui, api, infra) now owns its own Makefile, and the
root Makefile is a lean delegator for the base commands (run, build,
test, clean, fmt). Odin source, deps, and web assets live under gui/
(gui/src, gui/lib, gui/www), with scripts kept under gui/scripts for
build tooling only.

api/Makefile

@@ -0,0 +1,30 @@
+# api/Makefile — the Rust + axum backend API.
+#
+# Driven directly with `make -C api <target>` (or via the root Makefile's
+# `api-*` convenience targets).
+
+CARGO ?= cargo
+
+.PHONY: help run build test check fmt clean
+
+help: ## List available targets
+	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | \
+		awk 'BEGIN {FS = ":.*?## "}; {printf "  \033[36m%-18s\033[0m %s\n", $$1, $$2}'
+
+run: ## Run the API server (dev)
+	$(CARGO) run
+
+build: ## Release build -> target/release/
+	$(CARGO) build --release
+
+test: ## Run unit/integration tests
+	$(CARGO) test
+
+check: ## Format-check + lint (clippy -D warnings)
+	$(CARGO) fmt --all --check && $(CARGO) clippy --all-targets -- -D warnings
+
+fmt: ## Format Rust sources with rustfmt
+	$(CARGO) fmt
+
+clean: ## Remove Cargo build artifacts
+	$(CARGO) clean