Rewrite from Python to Go for single-binary cross-platform builds

Replaces imapdown.py with a multi-file Go implementation using
github.com/emersion/go-imap/v2. All features preserved: SSL/STARTTLS,
incremental UID-based downloads, attachment extraction to zip,
modified UTF-7 folder name decoding, and full-mode safety checks.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Makefile

@@ -0,0 +1,32 @@
+.PHONY: build build-all clean test install
+
+# Binary name
+BINARY=imapdown
+
+# Build flags for smaller binaries
+LDFLAGS=-ldflags="-s -w"
+
+# Build for current platform
+build:
+	go build $(LDFLAGS) -o $(BINARY)
+
+# Cross-compile for multiple platforms
+build-all:
+	GOOS=linux GOARCH=amd64 go build $(LDFLAGS) -o $(BINARY)-linux-amd64
+	GOOS=linux GOARCH=arm64 go build $(LDFLAGS) -o $(BINARY)-linux-arm64
+	GOOS=darwin GOARCH=amd64 go build $(LDFLAGS) -o $(BINARY)-darwin-amd64
+	GOOS=darwin GOARCH=arm64 go build $(LDFLAGS) -o $(BINARY)-darwin-arm64
+	GOOS=windows GOARCH=amd64 go build $(LDFLAGS) -o $(BINARY)-windows-amd64.exe
+
+# Clean build artifacts
+clean:
+	rm -f $(BINARY)
+	rm -f $(BINARY)-*
+
+# Run tests
+test:
+	go test -v ./...
+
+# Install to $GOPATH/bin
+install:
+	go install $(LDFLAGS)