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.js

@@ -721,6 +721,13 @@ let syncDirHandle = null;
 const SYNC_FILE_NAME = 'silent-send-sync.json';
 
 async function initFileSync() {
+  // Hide the entire section if File System Access API isn't supported
+  if (!window.showDirectoryPicker) {
+    const section = $('#fileSyncSection');
+    if (section) section.style.display = 'none';
+    return;
+  }
+
   syncDirHandle = await SilentSendSync.loadSyncDirHandle();
   updateFileSyncUI();
   if (syncDirHandle) {