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

package.json

@@ -2,7 +2,7 @@
   "name": "silent-send",
   "version": "0.3.1",
   "private": true,
-  "license": "MIT",
+  "license": "BSL-1.1",
   "description": "Browser extension that substitutes personal data before sending to AI services",
   "scripts": {
     "build:chrome": "./build.sh chrome",