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

LICENSE

@@ -1,21 +1,38 @@
-MIT License
+Business Source License 1.1
 
-Copyright (c) 2025 Silent Send Contributors
+Licensor: Silent Send Contributors
+Licensed Work: Silent Send browser extension
+Change Date: March 26, 2030
+Change License: MIT
 
-Permission is hereby granted, free of charge, to any person obtaining a copy
-of this software and associated documentation files (the "Software"), to deal
-in the Software without restriction, including without limitation the rights
-to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-copies of the Software, and to permit persons to whom the Software is
-furnished to do so, subject to the following conditions:
+Terms
 
-The above copyright notice and this permission notice shall be included in all
-copies or substantial portions of the Software.
+The Licensor hereby grants you the right to copy, modify, create
+derivative works, redistribute, and make non-production use of the
+Licensed Work.
 
-THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-SOFTWARE.
+The Licensor hereby grants you the right to make production use of
+the Licensed Work for personal, non-commercial purposes.
+
+For commercial use, you must obtain a commercial license from the
+Licensor. Contact: [your-email-here]
+
+Effective on the Change Date, the Licensor hereby grants you rights
+under the terms of the Change License, and the rights granted above
+terminate.
+
+If your use of the Licensed Work does not comply with the
+requirements currently in effect as described in this License, you
+must purchase a commercial license from the Licensor, or you must
+refrain from using the Licensed Work.
+
+All copies of the original and modified Licensed Work, and
+derivative works of the Licensed Work, are subject to this License.
+
+THE LICENSED WORK IS PROVIDED "AS IS". THE LICENSOR HEREBY DISCLAIMS
+ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
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+NONINFRINGEMENT. IN NO EVENT SHALL THE LICENSOR BE LIABLE FOR ANY
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