fix: chunk auto-backup writes + spinner on manual "Backup now"

Bridge.writeFile had the same URL-length trap saveFile did: a large
auto-backup base64'd into a single cmd://file/write URL blew past
WebView2's ~2MB navigation cap, so backups of big vaults failed
silently (or blanked the page) — for BOTH the manual "Backup now"
button and the silent scheduled run.

- writeFile now streams payloads over ~1MB in chunks (reusing the same
  file/chunk transport as saveFile), committed via a new
  file/write-commit. Delphi shares the decode+write logic through a new
  WriteDecodedFile helper and the existing FFileSaveChunks buffer.
- runAutoBackupNow(silent): the manual run shows the busy overlay
  ("Reading vault… N/total" → "Encrypting backup…" → "Writing file… N%")
  since a 20MB backup takes ~30s; the scheduled on-unlock run passes
  silent=true (no overlay, but still chunked so it no longer fails on
  large vaults).
- Fixed the "Backup now" click handler passing the click Event as the
  silent arg (truthy → would have suppressed the spinner and swallowed
  errors); wrapped in () => runAutoBackupNow().
- Pre-sync backup benefits from the chunked writeFile automatically.

Rebuild: BuildAssets + F9 (UMainForm.pas changed).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

delphi-backend/UMainForm.pas

@@ -115,6 +115,9 @@ type
     // (chunked): decode the base64, show the Save dialog, write, and fire
     // Bridge.onFileSaveResult back to JS.
     procedure SaveDecodedFile(const AName, AB64, AReqId: string);
+    // Silent write to a fixed path (auto-backup / sync pre-backup). Shared
+    // by cmd://file/write (single-shot) and file/write-commit (chunked).
+    procedure WriteDecodedFile(const APath, AB64, AReqId: string);
     procedure LogLine(const AMsg: string);
     procedure UpdateButtons;
     procedure NavigateToVault;
@@ -508,6 +511,40 @@ begin
     BoolToStr(LOk, True).ToLower + ',"' + LEscPath + '","' + LEscErr + '")');
 end;
 
+procedure TMainForm.WriteDecodedFile(const APath, AB64, AReqId: string);
+var
+  LOk: Boolean;
+  LErr: string;
+begin
+  LOk  := False;
+  LErr := '';
+  try
+    var LBytes := TNetEncoding.Base64.DecodeStringToBytes(AB64);
+    var LStream := TFileStream.Create(APath, fmCreate);
+    try
+      if Length(LBytes) > 0 then
+        LStream.WriteBuffer(LBytes[0], Length(LBytes));
+    finally
+      LStream.Free;
+    end;
+    LOk := True;
+    LogLine(Format('File written: %s (%d bytes)', [APath, Length(LBytes)]));
+  except
+    on E: Exception do
+    begin
+      LErr := E.Message;
+      LogLine('File write FAILED for "' + APath + '": ' + LErr);
+    end;
+  end;
+  var LEscReq := StringReplace(AReqId, '"', '\"', [rfReplaceAll]);
+  var LEscErr := StringReplace(LErr, '\', '\\', [rfReplaceAll]);
+  LEscErr := StringReplace(LEscErr, '"', '\"', [rfReplaceAll]);
+  WebBrowser.ExecuteJavaScript(
+    'if(window.Bridge&&Bridge.onFileWriteResult)' +
+    'Bridge.onFileWriteResult("' + LEscReq + '",' +
+    BoolToStr(LOk, True).ToLower + ',"' + LEscErr + '")');
+end;
+
 procedure TMainForm.NavigateToVault;
 begin
   FPendingURL := 'http://127.0.0.1:' + FServer.BoundPort.ToString + '/index.html';
@@ -1240,37 +1277,29 @@ begin
   // cmd://file/write?path=<full>&data=<base64>&reqId=<id>
   // Callback: Bridge.onFileWriteResult(reqId, ok, error)
   else if ACmd = 'file/write' then
+    // Single-shot silent write (small payload).
+    WriteDecodedFile(GetParam('path'), GetParam('data'), GetParam('reqId'))
+
+  // Chunked silent write: chunks accumulated via file/chunk, committed
+  // here (large auto-backups exceed a single cmd:// URL otherwise).
+  else if ACmd = 'file/write-commit' then
   begin
-    var LPath := GetParam('path');
-    var LData := GetParam('data');
     var LReqId := GetParam('reqId');
-    var LOk := False;
-    var LErr := '';
-    try
-      var LBytes := TNetEncoding.Base64.DecodeStringToBytes(LData);
-      var LStream := TFileStream.Create(LPath, fmCreate);
-      try
-        if Length(LBytes) > 0 then
-          LStream.WriteBuffer(LBytes[0], Length(LBytes));
-      finally
-        LStream.Free;
-      end;
-      LOk := True;
-      LogLine(Format('File written: %s (%d bytes)', [LPath, Length(LBytes)]));
-    except
-      on E: Exception do
-      begin
-        LErr := E.Message;
-        LogLine('File write FAILED for "' + LPath + '": ' + LErr);
-      end;
+    var LSB: TStringBuilder;
+    if FFileSaveChunks.TryGetValue(LReqId, LSB) then
+    begin
+      var LFull := LSB.ToString;
+      LSB.Free;
+      FFileSaveChunks.Remove(LReqId);
+      WriteDecodedFile(GetParam('path'), LFull, LReqId);
+    end
+    else
+    begin
+      var LEscReq := StringReplace(LReqId, '"', '\"', [rfReplaceAll]);
+      WebBrowser.ExecuteJavaScript(
+        'if(window.Bridge&&Bridge.onFileWriteResult)' +
+        'Bridge.onFileWriteResult("' + LEscReq + '",false,"no chunks buffered")');
     end;
-    var LEscReq := StringReplace(LReqId, '"', '\"', [rfReplaceAll]);
-    var LEscErr := StringReplace(LErr, '\', '\\', [rfReplaceAll]);
-    LEscErr := StringReplace(LEscErr, '"', '\"', [rfReplaceAll]);
-    WebBrowser.ExecuteJavaScript(
-      'if(window.Bridge&&Bridge.onFileWriteResult)' +
-      'Bridge.onFileWriteResult("' + LEscReq + '",' +
-      BoolToStr(LOk, True).ToLower + ',"' + LEscErr + '")');
   end
 
   // ---- Auto-backup: list files in dir matching name prefix --------------