feat: decrypt Jasypt values in .env and Spring files

ENC(...) values are decrypted on reveal, copy or get with
jasypt-spring-boot's default algorithm (3.0+, PBEWITHHMACSHA512ANDAES_256).

- The password is, as in Spring, $JASYPT_ENCRYPTOR_PASSWORD, else the
  file's own jasypt.encryptor.password (JASYPT_ENCRYPTOR_PASSWORD in a
  .env), with ${VAR} and ${VAR:default} read from the environment.
- Errors say which password was used, or which one is missing.
- Secrets show a detail after their name: jasypt, or reference for a
  Spring ${VAR}; also in Go to and Favourites.
- A .env whose values are all Jasypt isn't marked as plain text in git.

internal/jasypt/jasypt_test.go

@@ -0,0 +1,37 @@
+package jasypt
+
+import (
+	"errors"
+	"testing"
+)
+
+// Encrypted by Jasypt 1.9.3 itself, set up as jasypt-spring-boot's default
+// encryptor, with the password "test-password".
+var vectors = map[string]string{
+	"ENC(5IQUKT4B7j38TnZJ+BlWqo/zSkKHTjl4XRJyaQEHgkCy4r00avLt4dWqFB74X0Zw)":                         "hunter2",
+	"ENC(w4pm2JVZcbG5R+Ud9uKHg8VhJ/0aAK7sFXtNBv+Vo450SAchTQtfhw0TnIJ9gN+D0VFbwqJzfXU369coTJz2rw==)": "blåbærsyltetøy ✓",
+}
+
+func TestDecrypt(t *testing.T) {
+	for enc, want := range vectors {
+		if !Is(enc) {
+			t.Errorf("%s: not Jasypt", enc)
+		}
+		if got, err := Decrypt(enc, "test-password"); err != nil || got != want {
+			t.Errorf("%s: %q %v, want %q", enc, got, err, want)
+		}
+		if _, err := Decrypt(enc, "wrong"); !errors.Is(err, ErrDecrypt) {
+			t.Errorf("%s with a wrong password: %v", enc, err)
+		}
+	}
+	for _, v := range []string{"ENC(not base64!)", "ENC(aGVsbG8=)"} {
+		if _, err := Decrypt(v, "test-password"); err == nil {
+			t.Errorf("%s: no error", v)
+		}
+	}
+	for _, v := range []string{"ENC()", "hunter2", "ENC[AES256_GCM,data:x]", "${DB_PASSWORD}"} {
+		if Is(v) {
+			t.Errorf("%s: Jasypt", v)
+		}
+	}
+}