feat: BSL license + encrypted export/import for cross-browser transfer

License: Changed from MIT to BSL 1.1. Free for personal use,
commercial use requires a paid license. Auto-converts to MIT
on March 26, 2030.

Export/Import: Options page now has "Transfer Data" section:
- Export All (plain) — JSON file with all identities, mappings, settings
- Export Encrypted — AES-256-GCM with PBKDF2 password derivation,
  saved as .ssbackup file
- Import — handles both plain and encrypted backups, prompts for
  password if encrypted

Crypto uses Web Crypto API (browser-native, no dependencies):
100k PBKDF2 iterations, random salt + IV per export.

https://claude.ai/code/session_01Dvgwe7XMoSxnWXkih8p1Cw

src/lib/crypto.js

@@ -0,0 +1,92 @@
+/**
+ * Silent Send - Crypto Module
+ *
+ * AES-256-GCM encryption with PBKDF2 key derivation.
+ * Used for encrypted export/import of user data.
+ */
+
+const SALT_LENGTH = 16;
+const IV_LENGTH = 12;
+const ITERATIONS = 100000;
+
+async function deriveKey(password, salt) {
+  const encoder = new TextEncoder();
+  const keyMaterial = await crypto.subtle.importKey(
+    'raw',
+    encoder.encode(password),
+    'PBKDF2',
+    false,
+    ['deriveKey']
+  );
+
+  return crypto.subtle.deriveKey(
+    {
+      name: 'PBKDF2',
+      salt,
+      iterations: ITERATIONS,
+      hash: 'SHA-256',
+    },
+    keyMaterial,
+    { name: 'AES-GCM', length: 256 },
+    false,
+    ['encrypt', 'decrypt']
+  );
+}
+
+const SilentSendCrypto = {
+  /**
+   * Encrypt data with a password.
+   * Returns a base64 string containing salt + iv + ciphertext.
+   */
+  async encrypt(data, password) {
+    const encoder = new TextEncoder();
+    const salt = crypto.getRandomValues(new Uint8Array(SALT_LENGTH));
+    const iv = crypto.getRandomValues(new Uint8Array(IV_LENGTH));
+    const key = await deriveKey(password, salt);
+
+    const plaintext = encoder.encode(JSON.stringify(data));
+    const ciphertext = await crypto.subtle.encrypt(
+      { name: 'AES-GCM', iv },
+      key,
+      plaintext
+    );
+
+    // Combine: salt (16) + iv (12) + ciphertext
+    const combined = new Uint8Array(salt.length + iv.length + ciphertext.byteLength);
+    combined.set(salt, 0);
+    combined.set(iv, salt.length);
+    combined.set(new Uint8Array(ciphertext), salt.length + iv.length);
+
+    // Base64 encode
+    return btoa(String.fromCharCode(...combined));
+  },
+
+  /**
+   * Decrypt data with a password.
+   * Takes the base64 string from encrypt().
+   */
+  async decrypt(encryptedBase64, password) {
+    const combined = Uint8Array.from(atob(encryptedBase64), c => c.charCodeAt(0));
+
+    const salt = combined.slice(0, SALT_LENGTH);
+    const iv = combined.slice(SALT_LENGTH, SALT_LENGTH + IV_LENGTH);
+    const ciphertext = combined.slice(SALT_LENGTH + IV_LENGTH);
+
+    const key = await deriveKey(password, salt);
+
+    try {
+      const plaintext = await crypto.subtle.decrypt(
+        { name: 'AES-GCM', iv },
+        key,
+        ciphertext
+      );
+
+      const decoder = new TextDecoder();
+      return JSON.parse(decoder.decode(plaintext));
+    } catch (e) {
+      throw new Error('Wrong password or corrupted data');
+    }
+  },
+};
+
+export default SilentSendCrypto;