Initial commit: Phase 1+2 prototype

- Go backend with SQLite, JWT auth, file CRUD
- Vue 3 frontend with split/raw/WYSIWYG editor modes
- Markdown preview (marked, GFM)
- Formatting toolbar + keyboard shortcuts
- File tree with search, create, delete
- Light/dark theme toggle
- Admin panel (user management)
- Preferences (timezone, theme, default mode)
- Shared documents section (placeholder)
- Export: PDF, HTML, MD
- Build daemon (Python, stdlib only)
- Build job queue API
- Docker deployment

internal/crypto/crypto.go

@@ -0,0 +1,64 @@
+package crypto
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"errors"
+	"io"
+
+	"golang.org/x/crypto/argon2"
+)
+
+// DeriveKey derives a 256-bit key from password and salt using Argon2id.
+func DeriveKey(password []byte, salt []byte) []byte {
+	return argon2.IDKey(password, salt, 3, 64*1024, 4, 32)
+}
+
+// GenerateSalt returns a random 16-byte salt.
+func GenerateSalt() ([]byte, error) {
+	salt := make([]byte, 16)
+	_, err := io.ReadFull(rand.Reader, salt)
+	return salt, err
+}
+
+// Encrypt encrypts plaintext with AES-256-GCM using the given key.
+func Encrypt(plaintext, key []byte) ([]byte, error) {
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, err
+	}
+
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return nil, err
+	}
+
+	return gcm.Seal(nonce, nonce, plaintext, nil), nil
+}
+
+// Decrypt decrypts AES-256-GCM ciphertext using the given key.
+func Decrypt(ciphertext, key []byte) ([]byte, error) {
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, err
+	}
+
+	nonceSize := gcm.NonceSize()
+	if len(ciphertext) < nonceSize {
+		return nil, errors.New("ciphertext too short")
+	}
+
+	nonce, ciphertext := ciphertext[:nonceSize], ciphertext[nonceSize:]
+	return gcm.Open(nil, nonce, ciphertext, nil)
+}