feat: add WannPassts booking app with Go backend and Vue frontend

Initial project setup for a privacy-focused, self-hosted scheduling app:

- Go backend with JWT auth, SQLite storage, and chi router
- Calendar integrations via Google OAuth (FreeBusy), CalDAV, and ICS links
- Public booking pages with configurable slots and booking requests
- AES-256-GCM encryption for stored provider tokens
- Vue 3 + TypeScript frontend with Vite, Pinia, and Vue Router
- Docs, .gitignore, and .env.example for local development

backend/internal/crypto/secret.go

@@ -0,0 +1,52 @@
+package crypto
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"errors"
+	"io"
+)
+
+// Encrypt verschlüsselt plaintext mit AES-256-GCM (Key = SHA256(secret))
+// und liefert base64(nonce + ciphertext).
+func Encrypt(secret string, plaintext []byte) (string, error) {
+	k := sha256.Sum256([]byte(secret))
+	block, err := aes.NewCipher(k[:])
+	if err != nil {
+		return "", err
+	}
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", err
+	}
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return "", err
+	}
+	sealed := gcm.Seal(nonce, nonce, plaintext, nil)
+	return base64.StdEncoding.EncodeToString(sealed), nil
+}
+
+// Decrypt macht Encrypt rückgängig.
+func Decrypt(secret, s string) ([]byte, error) {
+	data, err := base64.StdEncoding.DecodeString(s)
+	if err != nil {
+		return nil, err
+	}
+	k := sha256.Sum256([]byte(secret))
+	block, err := aes.NewCipher(k[:])
+	if err != nil {
+		return nil, err
+	}
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, err
+	}
+	if len(data) < gcm.NonceSize() {
+		return nil, errors.New("ciphertext zu kurz")
+	}
+	return gcm.Open(nil, data[:gcm.NonceSize()], data[gcm.NonceSize():], nil)
+}