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/options/options.html

@@ -66,13 +66,24 @@
       </div>
     </section>
 
+    <section class="section">
+      <h2>Transfer Data</h2>
+      <p class="section-desc">Export all your identities, mappings, and settings to move between browsers. Encrypted exports require a password to decrypt.</p>
+      <div class="bulk-actions">
+        <button class="btn" id="btnExportAll">Export All (plain)</button>
+        <button class="btn" id="btnExportEncrypted">Export Encrypted</button>
+        <button class="btn" id="btnImportAll">Import</button>
+        <input type="file" id="fileImportAll" accept=".json,.ssbackup" hidden>
+      </div>
+    </section>
+
     <section class="section">
       <h2>Mappings</h2>
       <p class="section-desc">Manage all your substitution rules. Longer matches take priority.</p>
 
       <div class="bulk-actions">
-        <button class="btn" id="btnExport">Export JSON</button>
-        <button class="btn" id="btnImport">Import JSON</button>
+        <button class="btn" id="btnExport">Export Mappings</button>
+        <button class="btn" id="btnImport">Import Mappings</button>
         <input type="file" id="fileImport" accept=".json" hidden>
         <button class="btn btn-danger" id="btnClearAll">Clear All</button>
       </div>