refactor: consolidate blockchain explorer into single app and update backup ignore patterns

- Remove standalone explorer-web app (README, HTML, package files)
- Add /web endpoint to blockchain-explorer for web interface access
- Update .gitignore to exclude application backup archives (*.tar.gz, *.zip)
- Add backup documentation files to .gitignore (BACKUP_INDEX.md, README.md)
- Consolidate explorer functionality into main blockchain-explorer application

apps/wallet/src/app/crypto/encryption.py

@@ -0,0 +1,66 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Optional
+
+from argon2.low_level import Type as Argon2Type, hash_secret_raw
+from nacl.bindings import (
+    crypto_aead_xchacha20poly1305_ietf_decrypt,
+    crypto_aead_xchacha20poly1305_ietf_encrypt,
+)
+
+from ..security import wipe_buffer
+
+
+class EncryptionError(Exception):
+    """Raised when encryption or decryption fails."""
+
+
+@dataclass
+class EncryptionSuite:
+    """Argon2id + XChaCha20-Poly1305 helper functions."""
+
+    salt_bytes: int = 16
+    nonce_bytes: int = 24
+    key_bytes: int = 32
+    argon_time_cost: int = 3
+    argon_memory_cost: int = 64 * 1024  # kibibytes
+    argon_parallelism: int = 2
+
+    def _derive_key(self, *, password: str, salt: bytes) -> bytes:
+        password_bytes = password.encode("utf-8")
+        return hash_secret_raw(
+            secret=password_bytes,
+            salt=salt,
+            time_cost=self.argon_time_cost,
+            memory_cost=self.argon_memory_cost,
+            parallelism=self.argon_parallelism,
+            hash_len=self.key_bytes,
+            type=Argon2Type.ID,
+        )
+
+    def encrypt(self, *, password: str, plaintext: bytes, salt: bytes, nonce: bytes) -> bytes:
+        key = self._derive_key(password=password, salt=salt)
+        try:
+            return crypto_aead_xchacha20poly1305_ietf_encrypt(
+                message=plaintext,
+                aad=b"",
+                nonce=nonce,
+                key=key,
+            )
+        except Exception as exc:  # pragma: no cover
+            raise EncryptionError("encryption failed") from exc
+
+    def decrypt(self, *, password: str, ciphertext: bytes, salt: bytes, nonce: bytes) -> bytes:
+        key_bytes = bytearray(self._derive_key(password=password, salt=salt))
+        try:
+            return crypto_aead_xchacha20poly1305_ietf_decrypt(
+                ciphertext=ciphertext,
+                aad=b"",
+                nonce=nonce,
+                key=bytes(key_bytes),
+            )
+        except Exception as exc:
+            raise EncryptionError("decryption failed") from exc
+        finally:
+            wipe_buffer(key_bytes)