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>

state.go

@@ -0,0 +1,65 @@
+package main
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+)
+
+const stateFileName = ".imapdown_state.json"
+
+// State tracks the highest UID downloaded per folder
+type State map[string]uint32
+
+// LoadState reads the state file from the base directory
+// Returns empty state if file doesn't exist or can't be read
+func LoadState(baseDir string, fullMode bool) (State, error) {
+	if fullMode {
+		return make(State), nil
+	}
+
+	statePath := filepath.Join(baseDir, stateFileName)
+
+	data, err := os.ReadFile(statePath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return make(State), nil
+		}
+		return make(State), err
+	}
+
+	var state State
+	if err := json.Unmarshal(data, &state); err != nil {
+		// Return empty state on parse error
+		return make(State), nil
+	}
+
+	return state, nil
+}
+
+// SaveState writes the state file to the base directory with indentation
+func SaveState(baseDir string, state State) error {
+	statePath := filepath.Join(baseDir, stateFileName)
+
+	data, err := json.MarshalIndent(state, "", "  ")
+	if err != nil {
+		return err
+	}
+
+	return os.WriteFile(statePath, data, 0644)
+}
+
+// UpdateFolder updates the highest UID for a folder
+func (s State) UpdateFolder(folder string, uid uint32) {
+	if current, exists := s[folder]; !exists || uid > current {
+		s[folder] = uid
+	}
+}
+
+// GetLastUID returns the last UID for a folder, or 0 if not found
+func (s State) GetLastUID(folder string) uint32 {
+	if uid, exists := s[folder]; exists {
+		return uid
+	}
+	return 0
+}