fix: sync code not transferring identity when encryption enabled

_getAllData() was reading raw storage which contains encrypted blobs
when at-rest encryption is enabled. The sync code export would send
{ _ssLocalEncrypted: true, data: <blob> } instead of actual identity
data. The importing browser couldn't use these blobs.

Fixed _getAllData() to use Storage._readSecure() which decrypts
transparently. Fixed _applyData() to use Storage._writeSecure()
so imported data gets encrypted on the receiving end.

Also: hide the Auto-Sync Folder section entirely in browsers that
don't support File System Access API (Firefox, Brave, Safari)
instead of showing a broken-looking error message.

https://claude.ai/code/session_01SWSwDfMVij53bCTNSCLMwn

src/options/options.html

@@ -238,7 +238,7 @@
         <div id="syncStatus" style="margin-top:8px;font-size:12px;min-height:16px"></div>
       </div>
 
-      <div style="margin-top:20px;padding-top:16px;border-top:1px solid #e5e7eb">
+      <div id="fileSyncSection" style="margin-top:20px;padding-top:16px;border-top:1px solid #e5e7eb">
         <h3 style="font-size:13px;font-weight:600;margin:0 0 4px">Auto-Sync Folder — fully automatic, no copy-paste</h3>
         <p class="section-desc" style="margin-bottom:8px">
           Pick the same folder in each browser once. Changes are written to